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\begin{document}

\title{LOW-HEAD DAM REMOVAL INCREASES FUNCTIONAL DIVERSITY OF STREAM FISH
ASSEMBLAGES}



\author[1]{Adam Jones}%
\author[2]{Scott Meiners}%
\author[2]{EFFERT-FANTA EDEN}%
\author[3]{Trent Thomas}%
\author[4]{C.F. SHANNON SMITH}%
\author[2]{Robert Colombo}%
\affil[1]{US Geological Survey Columbia Environmental Research Center}%
\affil[2]{Eastern Illinois University Department of Biological Sciences}%
\affil[3]{Illinois Department of Natural Resources}%
\affil[4]{William \& Mary Virginia Institute of Marine Science}%


\vspace{-1em}



  
  \date{June 16, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Despite the growing number of dam removals, very few have been studied
to understand their impacts on stream fish communities. An even smaller
proportion of dam removal studies focus on the impacts of low-head dam
removals, although they are the most common type of dam. Instead, the
majority of removal studies focus on the impacts of larger dams. In this
study, two previously impounded Illinois rivers were monitored to assess
the impacts of low-head dam removal on the functional assemblage of
stream fishes. Study sites were sampled each fall from 2012-2015
(pre-dam removal) and 2018-2020 (post-dam removal) in three habitat
types: downstream of the dam, impounded areas, and runs of rivers.
Fishes were aggregated into habitat and reproductive guilds, relating
community changes to habitat, environmental metrics, and stream quality.
Prior to removal, the slackwater guild was the most prevalent habitat
guild throughout both rivers, while nest builders and benthic spawners
were the most abundant reproductive guilds. During the two years
following removal, habitat conditions and fish assemblages improved
throughout both rivers, with improvements in QHEI, IBI, water
temperature, and dissolved oxygen, as well as a shift to more evenly
distributed representation of habitat and reproductive guilds. The
improvements in environmental metrics and overall stream quality,
particularly in the formerly impounded habitats, indicate diminished
habitat homogeneity, and a shift towards natural habitat diversity. This
habitat diversification likely led to the restoration of a range of
potential niches, thereby increasing the array of guild types inhabiting
these rivers, while simultaneously preventing single-guild dominance.%
\end{abstract}%



\sloppy


\textbf{LOW-HEAD DAM REMOVAL INCREASES FUNCTIONAL DIVERSITY OF STREAM
FISH ASSEMBLAGES}

ADAM C. JONES\textsuperscript{a*}, SCOTT J. MEINERS\textsuperscript{b},
EDEN EFFERT-FANTA\textsuperscript{b}, TRENT THOMAS\textsuperscript{c} ,
SHANNON C.F. SMITH\textsuperscript{d}, AND ROBERT E.
COLOMBO\textsuperscript{b}

\textsuperscript{a} \emph{U.S. Geological Survey, Columbia Environmental
Research Center, Columbia, Missouri, USA}

\emph{\textsuperscript{b} Department of Biological Sciences, Eastern
Illinois University, Charleston, Illinois, USA}

\emph{\textsuperscript{c} Illinois Department of Natural Resources, One
Natural Resources Way, Springfield, Illinois, USA}

\emph{\textsuperscript{d} Department of Fisheries
Science,\textbf{~}Virginia Institute of Marine Science, William \&
Mary,Gloucester Point, Virginia, USA}

[?] Correspondence to: Adam C. Jones, U.S. Geological Survey, Columbia
Environmental Research Center, 4200 East New Haven Road, Columbia,
Missouri, 65201, USA. Email: acjones@usgs.gov

ACKNOWLEDGEMENTS

The authors would like to thank the graduate and undergraduate students
at Eastern Illinois University's Fisheries and Aquatic Sciences Research
Team for their tremendous contributions to the fieldwork involved with
this study. This Sport Fish Restoration research funding was provided in
whole or in part by the U.S. Fish and Wildlife Service in conjunction
with the Illinois Department of Natural Resources (F-186-R2).

ABSTRACT

Despite the growing number of dam removals, very few have been studied
to understand their impacts on stream fish communities. An even smaller
proportion of dam removal studies focus on the impacts of low-head dam
removals, although they are the most common type of dam. Instead, the
majority of removal studies focus on the impacts of larger dams. In this
study, two previously impounded Illinois rivers were monitored to assess
the impacts of low-head dam removal on the functional assemblage of
stream fishes. Study sites were sampled each fall from 2012-2015
(pre-dam removal) and 2018-2020 (post-dam removal) in three habitat
types: downstream of the dam, impounded areas, and runs of rivers.
Fishes were aggregated into habitat and reproductive guilds, relating
community changes to habitat, environmental metrics, and stream quality.
Prior to removal, the slackwater guild was the most prevalent habitat
guild throughout both rivers, while nest builders and benthic spawners
were the most abundant reproductive guilds. During the two years
following removal, habitat conditions and fish assemblages improved
throughout both rivers, with improvements in QHEI, IBI, water
temperature, and dissolved oxygen, as well as a shift to more evenly
distributed representation of habitat and reproductive guilds. The
improvements in environmental metrics and overall stream quality,
particularly in the formerly impounded habitats, indicate diminished
habitat homogeneity, and a shift towards natural habitat diversity. This
habitat diversification likely led to the restoration of a range of
potential niches, thereby increasing the array of guild types inhabiting
these rivers, while simultaneously preventing single-guild dominance.

KEY WORDS: low-head dam; dam removal; reproductive guilds; habitat
guilds; diversity; functional composition; habitat restoration

INTRODUCTION

To date, more than 1,400 dams have been removed from U.S. waterways
(American Rivers 2019), however, less than 10\% of removals have been
studied to understand their impacts on stream fishes (Bellmore \emph{et
al.} 2017). Stream fishes are often particularly vulnerable to the
ecological impacts imposed by dams (Oliveira \emph{et al.} 2018;
Turgeon\emph{et al.} 2019; Barbarossa \emph{et al.} 2020), often
resulting in significant shifts in community composition, such as an
increase in homogenization of assemblages in streams with the higher
spring flow (Hastings \emph{et al.} 2016). Such changes in fish
communities may arise from many drivers, including fragmentation of
populations, altered hydrology and flow regime, reduced lateral exchange
of sediments and nutrients, and alteration of biological and physical
characteristics of the river channel and flood plain (Bednarek 2001). Of
these impairments, the shift from lotic to lentic conditions is
particularly problematic to many stream fishes. Such a shift often
results in dominance of fishes adapted to lentic conditions and those
possessing a high degree of functional plasticity, as they are capable
of inhabiting lacustrine conditions (Agostinho \emph{et al.} 2008;
Turgeon \emph{et al.} 2019). As sufficient plasticity and tolerance is
not common in fishes, reductions in diversity and abundance are often
associated with impounded systems (Agostinho \emph{et al.} 2008; Turgeon
\emph{et al.}2019).

Although dam removals are often motivated by dam age and degradation
that diminish utility (Doyle \emph{et al.} 2003), increased public
awareness of the ecological costs imposed by dams and the desire to
restore rivers to a more natural state are also driving forces (Bednarek
2001; Poulos \emph{et al.} 2014; Poulos \& Chernoff 2016). Despite the
intent to restore the system, successful outcomes are uncertain (e.g.,
Cheng \& Granata 2007; Stanley \emph{et al.} 2007; Chang \emph{et
al.}2016). For example, dam removal would allow accumulated sediments to
move downstream, resulting in altered channel morphology, habitat
conditions, and nutrient transport (Hart \emph{et al.} 2002),
potentially degrading downstream conditions. Removing impoundments may
also reestablish natural flow regimes and facilitate movement of
migratory fauna, resulting in genetic or compositional changes
(Hart\emph{et al.} 2002; Catalano \emph{et al.} 2007; Haponski \emph{et
al.}2007; Ding \emph{et al.} 2019). Limited research examining
ecological shifts as a result of dam removal and the impacts on stream
fishes causes uncertainty in whether dam removal will be a beneficial or
detrimental course of action.

In assessing the relationship between dams and stream fishes, priority
has been given to larger dams at least 15 m high or that impound 3
million m\textsuperscript{3} of water (ICOD 2011). Very few studies have
focused on low-head dams (no higher than 9 m) although they make up the
majority of dams in the U.S. (USACE NID 2018; Iowa Department of Natural
Resources 2021). Within the limited studies on low-head dams, few
analyze the impacts of removal and the responses of stream fish
communities (Bellmore \emph{et al.} 2017). Rather, these studies focus
on understanding the effects on stream fishes and the environment in
response to the presence of dams (Butler \& Wahl 2010; Alexandre \&
Alemida 2010; Smith \emph{et al.} 2017). With the increasing rate of
removals (Poff \& Hart 2002), improved understanding of how low-head dam
removal affects stream fish assemblages is imperative.

Although a few studies document low-head dam removal and their effects
on fishes (Burdick \& Hightower 2006; Catalano \emph{et al.} 2007; Cook
\& Sullivan 2018), an even smaller proportion analyze functional impacts
on stream fish communities by examining guilds (Dorobek \emph{et
al.}2015; Ding \emph{et al.} 2019). Functional guilds can serve as
indicators of community response to variations in a river's hydrology,
geomorphology, and habitat structure (Welcomme \emph{et al.} 2006).
Because many of the factors used to aggregate fishes into guilds are
often impacted by dam presence and removal, changes in guild structure
should provide functional understanding. This approach emphasizes
connections between community composition and environmental parameters
such as increased abundance of pelagophils in response to improved
connectivity or increased abundance of riffle and run species in
response to increased lotic habitat. Despite its potential, use of a
guild structure may be complicated by limited data, intraspecific
variation, and ontogenetic shifts in functional traits (Benoit \emph{et
al.} 2021). Regardless, a guild structure offers an innovative and
versatile method to increase our understanding of stream fish community
dynamics (Benoit \emph{et al.} 2021).

While guilds provide an assessment of functional composition,
understanding environmental relationships may be strengthened by the
simultaneous use of additional metrics. The index of biotic integrity
(IBI; Karr 1981) is one such approach. IBI computes an index of stream
quality by integrating various aspects of fish communities (i.e.,
proportion of reproductive and feeding groups), as well as observed
environmental conditions, and comparing them to expected conditions of a
similar, undisturbed river or stream (Karr 1981; Oberdorff \& Hughes
1992). Because several attributes analyzed by IBI are synonymous with
those examined in a guild structure, utilizing the techniques in
conjunction will emphasize trends in functional composition in response
to potential environmental shifts following dam removal.

Given the paucity of functional assessments of dam removals, we utilized
habitat and reproductive guilds to analyze changes in the fish
communities of two Illinois streams in response to low-head dam removal.
To accomplish this, we analyzed fish and habitat data collected over 7
years; 4 years of pre-removal data (Hastings \emph{et al.} 2015, 2016;
Smith \emph{et al.} 2017) and 3 years of post-removal data, in an effort
to; (i) document immediate habitat and stream fish responses to low-head
dam removal (ii) document functional changes in stream fishes in
response to low-head dam removal and (iii) identify relationships
between environmental and stream fish responses. We expected that
overall health of the rivers would improve, with increased flow rates
and dissolved oxygen in response to dam removal, but the greatest
improvements would occur in the impounded reaches. We also predicted
that dominance of lacustrine-adapted fishes would decrease, increasing
functional group diversity within these rivers.

METHODS

\emph{Study Site}

This study analyzed two tributaries of the Wabash River located near
Danville, Illinois: the Vermilion River and the North Fork Vermilion
River (Figure 1). Both rivers were impounded by low-head dams located in
Danville since the early 1900s, until they were removed in 2018 (IDNR
2018). The Danville Dam was the furthest downstream impoundment on the
Vermilion River, located between the lower 35 km of the river and the
remaining 3,341 km\selectlanguage{ngerman}² upstream drainage area. The Ellsworth Park Dam was
located on the North Fork Vermilion River, about 4 km downstream of Lake
Vermilion, and just upstream of the confluence of the two rivers (IDNR
2018). Sampling took place at six study sites within each river, each
measuring 100 m in length. The six sites within each river consisted of
three habitat types: two downstream of the dam (DWN), two within
impounded areas (IMP) and two within the runs of the rivers (ROR)
(Figure 1; Hastings \emph{et al.} 2015). Pre-removal sampling occurred
in the fall of 2012-2015 and post-removal sampling occurred in the fall
of 2018-2020, except in the North Fork Vermilion River where sampling
did not occur in 2018 as the timing of the dam removal conflicted with
sampling events.

\emph{Fish Sampling}

Fish collection was conducted using DC electrofishing methods as
described in Hastings \emph{et al.} (2015); by boat on the Vermilion
River and barge on the North Fork Vermilion River, where waters levels
were too shallow for boat navigation, except in 2014 and 2015 where
elevated water levels required boat electrofishing. Each site was
sampled for 30 minutes, and fish were identified to species, weighed (g)
and measured (total length, mm) after each effort. Any specimen with a
total length below 100 mm was not weighed, and those that were not
easily identified in the field (e.g., \emph{Cyprinella} ) were
euthanized and preserved in 95\% ethanol to be identified in the lab.
Two species of redhorse inhabiting these rivers, the Black Redhorse and
Golden Redhorse (\emph{Moxostoma duquesni} and \emph{Moxostoma
erythrurum} , respectively) are not easily distinguished. Because of
this similarity, these species were photographed and released;
photographs were later examined to count lateral line scales to
distinguish these species (Golden Redhorse = 39-43 scales and Black
Redhorse=44-47 scales).

\emph{Assessment of Stream Health}

Stream health was evaluated by analyzing abiotic factors using Ohio
Qualitative Habitat Evaluation Index scores (QHEI; Rankin 2006) and by
analyzing biotic factors via Index of Biotic Integrity scores for each
site (IBI; Karr 1981; Smogor 2000). Six variables are utilized to
compute QHEI: substrate, instream cover, channel morphology, riparian
zone and bank erosion, pool/glide and riffle/run quality, and gradient,
designating a score to each. Metric scores are then summed to compute an
overall score. IBI is calculated using ten biotic metrics, including
number of native fish species, number of intolerant species, proportion
of tolerant species, and the proportions of several reproductive and
feeding groups. Each metric is then adjusted based on wetted stream
width and, similar to QHEI, summed to compute an overall score (Smogor
2000).

Water quality was also measured at every sampling event, from the
thalweg of each site, with a YSI Professional Plus (YSI Incorporated,
Yellow Springs, OH). The YSI meter recorded water temperature (C°),
specific conductivity (\selectlanguage{greek}µ\selectlanguage{english}S/cm), dissolved oxygen (mg/L), and pH. Other
variables assessed include surface water velocity (m/s) in the middle of
the channel (Hach Portable Velocity Meter; Hach Company, Loveland, CO),
turbidity (m; Secchi board), and stream width (m).

\emph{Guild Assignment}

Five habitat guilds were constructed following Spurgeon \emph{et
al.}(2019) and literature (Pflieger 1997; Page \& Burr 2011): lobate
margin, run, riffle, slackwater and habitat generalist (Table A1). The
lobate margin guild was described as including fishes that inhabit areas
of low velocity and shallow depths on channel margins. The run guild
included fishes that are most often found in the main channel, where
depths and velocities tend to be greater. The riffle guild was
characterized by fishes found in clearer waters, with slightly lower
velocities than main channels and containing coarse substrate. Fishes
belonging to the slackwater guild were those preferring off channel
pools or backwaters near stream edges. Finally, habitat generalist
fishes were those that are not associated with a specific habitat type
and are found in several types of habitats (Spurgeon \emph{et al} .
2019).

Reproductive guilds were constructed following Simon (1999), which is a
modified classification based on Balon (1975, 1981). The reproductive
guilds used here include: pelagophils, benthic spawners, brood hiders,
nest builders, and live bearers (Table A1). However, only one species of
live bearer, the Mosquitofish (\emph{Gambusia affinis} ), occurred in
our study and was only collected in the North Fork Vermilion prior to
dam removal. The reproductive guilds used were modified to group several
different guilds into more generalized ones, following Smith \emph{et
al.} (2017). For instance, all `guarder' sub-groups described by Balon
(1975, 1981) and Simon (1999) were included as `nest builders' in our
study.

\emph{Data Analysis}

Data used for each analysis consisted of catch per unit effort (CPUE;
fish/hr) using species abundance aggregated into habitat and
reproductive guilds. The resulting CPUE values were log + 1 transformed
to down-weight abundant taxa. All environmental data, water temperature,
dissolved oxygen, QHEI and IBI were log transformed, except for flow,
which was log + 1 transformed to address zeros in the data set. Two-way
analysis of variance (ANOVA), followed by Tukey's honest significant
difference (HSD) post-hoc tests, were used to determine impacts of dam
removal (pre- and post-removal) and habitat (downstream of dam,
impounded, run of river) that may explain trends in functional
composition, QHEI, IBI, environmental variables and overall species
abundances. River was included as a blocking variable to control for
variation between the two systems. Significant factors were subsequently
tested to identify differences using a Tukey HSD test.

Trends in functional composition associated with dam removal were
examined using nonmetric multidimensional scaling (NMDS) ordinations
using Bray-Curtis dissimilarity. The guild/site matrices used to compose
the NMDS consisted of the log + 1 transformed CPUE grouped by guild type
(habitat or reproductive). The relationships of guild and overall stream
health (QHEI and IBI) were related to the ordinations by plotting a
series of vectors. Significance of functional responses to habitat and
dam removal were assessed by a permutational multivariate analysis of
variance (PERMANOVA) separately for habitat and reproductive guilds,
again including river as a blocking factor. PERMANOVAs utilized
Bray-Curtis dissimilarity, consisted of 10,000 permutations and were
conducted with the \emph{adonis} command in the \emph{vegan} package of
R.

To assess the impacts of dam removal on guild assemblage, the
Shannon-Wiener diversity index (Shannon and Weaver, 1949), and abundance
of each functional group were analyzed using two-way ANOVAs as described
above. Diversity was calculated using the same log + 1 transformed data
described in the NMDS ordination above and CPUE of each functional group
was calculated. Live bearers were omitted from the abundance analysis
because of low occurrence in the dataset. R version 3.6 (R Foundation
for Statistical Computing) was used for all analyses.

RESULTS

\emph{Stream Health}

Water temperature and dissolved oxygen level were significantly impacted
by dam removal (Table 1), with water temperatures decreasing and
dissolved oxygen increasing following removal (Figure 2). However,
neither parameter differed among rivers or locations. Flow was
significantly higher in the Vermilion River as well as in the downriver
and run of river habitats (Figure 2) but showed no change in response to
dam removal (Table 1).

Neither QHEI nor IBI differed between rivers, but both varied
significantly among locations (Table 1; Figure 3), with both QHEI and
IBI highest in the run of river habitats, and lowest in the impounded
habitats. IBI also increased significantly following dam removal (Table
1; Figure 4). QHEI overall increased following dam removal but the
changes were largest in impounded reaches, resulting in a significant
location \selectlanguage{ngerman}× removal interaction (Table 1; Figure 3). Similarly, IBI
increased following dam removal in all sites, but impounded regions
experienced greater improvements than the other habitats (Figure 4).

\emph{Overall Abundances}

Following dam removal, abundances of fishes increased throughout both
rivers. (ANOVA F\textsubscript{1,71} = 15.27, P = 0.0002; Figure 5)
increasing at all sites, particularly in the impounded reaches.
Abundance of fishes also responded to location (ANOVA
F\textsubscript{2,71} = 5.29 , P = 0.007), with abundance being lowest
in the impounded reaches. Although abundance increased most
substantially in the impounded reaches, there was no location by removal
interaction (ANOVA F\textsubscript{2,71} = 1.74 , P = 0.182). Abundance
also differed between rivers (ANOVA F\textsubscript{1,71} = 11.24, P =
0.001) and were overall higher in the North Fork Vermilion.

\emph{Functional Assemblages and Guild Diversity}

There was a clear impact of dam removal on habitat guild composition in
both rivers as well as compositional differences between rivers and
among locations, which can be visualized in the NMDS ordination on
habitat guild composition (Table 2; Figure 6). In both rivers dam
removal resulted in a marked negative shift on NMDS1, reflecting the
decrease in slackwater and lobate margin guilds, and an increase in
riffle, run and habitat generalist guilds. Homogeneity of habitat guild
composition increased across all sites following dam removal. (Figure
6).

Reproductive guild composition within both rivers was clearly impacted
by dam removal ( Table 2: Figure 6). As with habitat guilds,
reproductive guild composition differed between rivers, among habitats,
and with dam removal. Again, there was no interaction between location
and dam removal, indicating system-wide compositional changes. These
compositional changes can be visualized in the NMDS ordination on
reproductive guild composition (Figure 6). Live bearers, nest builders
and to a lesser extent brood hiders were positively loaded on NMDS2 and
negatively loaded on NMDS1. Benthic spawners and pelagophils were
negatively loaded on both NMDS1 and NMDS2. Dam removal resulted in a
general negative shift along NMDS1 reflecting an increase in benthic
spawners and brood hiders with pelagophils also influencing the shift to
a lesser degree. Reproductive guild composition became more homogenous
across all habitats following dam removal (Figure 6).

QHEI and IBI were strongly related to the observed changes in functional
composition. QHEI was most strongly associated with riffle specialists
and, to a lesser extent, the run habitat guilds (Figure 6). In the
ordination of reproductive guilds, QHEI was strongly related to the
abundance of benthic spawners and independent from nest builders and
live bearers. Guilds associated with QHEI were similarly associated with
IBI, however these relationships were stronger. Within reproductive
guilds, QHEI and IBI were nearly identical in their guild relationships.
Both QHEI and IBI reflected the compositional changes associated with
dam removal, regardless of guild type.

Diversity of habitat and reproductive guilds increased following dam
removal by location (Table 3; Figure 7). Effects of dam removal varied
significantly by location in both guild types, however, only habitat
guild diversity differed between rivers, with higher diversity in the
North Fork Vermilion. While increases in the diversity of both guild
types were greatest within impounded reaches, a significant location by
removal interaction only occurred in reproductive guild diversity (Table
3).

\emph{Responses of Individual Guilds}

All habitat guilds responded to location based on their habitat
preferences and all guilds, except the run guild, differed between
rivers (Table 4; Figure A1). Dam removal increased the abundance of all
habitat guilds, except for the slackwater guild which did not change.
Although the interaction between location and dam removal was
non-significant in all habitat guilds, abundance of all guilds, except
slackwater, increased most substantially within the impounded reaches.

Reproductive guild abundance differed between rivers for all guilds,
except benthic spawners (Table 4; Figure A2). Abundance of all guilds,
except nest builders, differed among habitats and in response to dam
removal, with the greatest increases occurring in the impounded reaches
for these guilds. Brood hiders and pelagophils experienced the greatest
increases in abundance following dam removal. Additionally, the only
reproductive guild to exhibit an interaction between location and dam
removal was the pelagophils, which were restricted to the downstream
reaches of the Vermilion prior to dam removal but became widespread
across the entire system following dam removal (Table 4; Figure A2).

DISCUSSION

\emph{Stream Health}

Consistent with past studies, ecological and habitat conditions improved
throughout both rivers following dam removal (Kanehl \emph{et al} .1997;
Catalano \emph{et al.} 2007; Burroughs \emph{et al.} 2010; Butler and
Wahl 2010; Dorobek \emph{et al.} 2015). Although not significantly
different, flow rates increased in most locations following dam removal,
as would be expected in the absence of a physical barrier. Water
temperature decreased in both rivers following dam removal. This is
commonplace in dam removals, as lacustrine environments readily stratify
due to high surface area and low streamflow (Bednarek 2001;
Foley\emph{et al.} 2017). Likely associated with the combined alteration
in streamflow and water temperature, dissolved oxygen levels increased
substantially throughout both rivers (Gotovtsev 2010; Zhang \emph{et
al.} 2014).

QHEI scores following dam removal indicated an overall improvement in
stream condition. Impounded reaches were the poorest quality habitats in
both rivers pre-removal and despite experiencing an increase, retained
this status following dam removal. These locations may continue to
improve as seasonal flows reestablish more natural conditions.
Conversely, the runs of both rivers were the highest quality habitats
both before and after dam removal. IBI experienced a similar increase
following dam removal. Improvements in IBI were driven mainly by an
increase in intolerant species (e.g., Smallmouth Bass, Spotted Bass,
Spotted Sucker, Black Redhorse) and a decrease in tolerant species(e.g.,
Green Sunfish, Golden Redhorse) particularly in the North Fork Vermilion
River. Such shifts in tolerant and intolerant species congruent with
improved QHEI scores following dam removal are common (Hilsenhoff 1987;
Kanehl \emph{et al.} 1997; Stanley \emph{et al.} 2002; Catalano \emph{et
al.} 2007). Restoration of physical habitat (i.e., natural flow regime)
in the Vermilion and North Fork Vermilion Rivers likely facilitated the
success of intolerant species by promoting critical habitat components
of intolerant species' life history, such as spawning substrate, forage
base, or shelter.

\emph{Habitat Guilds}

Functional composition within both rivers in this study shifted
considerably with dam removal. Prior to removal, lentic-preferring
guilds were most prevalent throughout this system, particularly the
slackwater guild. The high abundance of this guild prior to dam removal
is unsurprising considering these fishes are characterized by an
affinity to lacustrine conditions, such as those imposed by dams
(Spurgeon \emph{et al.} 2019). Following dam removals, abundance of
nearly all guilds increased throughout both rivers, but the impounded
regions experienced the most dramatic increases. Prior to dam removal,
slackwater and habitat generalist guilds dominated impounded reaches.
However, compositional diversity increased substantially following dam
removal with more equal representation across guilds. Dam removal also
increased compositional diversity in the downriver and run of river
reaches, but to a lesser degree than impounded areas.

Stream fish assemblages are strongly dependent on physical habitat
(e.g., stream depth, flow, temperature), diversifying as conditions
improve (Gorman \& Karr 1978; Schlosser 1982; Rahel \& Hubert 1991;
Catalano \emph{et al.} 2007). Dams often degrade these conditions,
particularly by accumulating sediments, leading to habitat
homogenization, and eliminating distinctions between riffle, run and
pool fish communities (Berkman \& Rabeni 1987; Walling \& Amos 1999;
Collins \& Walling 2007; Kemp et al. 2011). Following dam removal,
sediment transport is commonly increased (Pawloski \& Cook 1993;
Kanehl\emph{et al.} 1997; Hart \emph{et al.} 2002; Burroughs \emph{et
al.}2010). While sediment transport was not measured in this system, it
is likely to have been stimulated by the connectivity. Nagayama \emph{et
al.} (2020) documented that increased sediment transport following dam
removal improved critical fish habitat and structure. Similarly, habitat
conditions in the current study improved throughout the rivers,
increasing the abundance of lotic guilds and heterogeneity of habitat
guild distribution.

\emph{Reproductive Guilds}

Similar to habitat guilds, reproductive guild diversity also underwent
stark transformations following dam removal. Nest builders and benthic
spawners dominated both rivers prior to dam removal and remain present
in large numbers even after dam removal. The nest builder guild was also
the only reproductive guild that did not experience a significant
increase following dam removal. Brood hiders, benthic spawners and
pelagophils experienced the greatest increases following removal. This
is unsurprising as dams inhibit the flow and connectivity essential to
pelagophil reproduction (Durham \& Wilde 2009; Mollenhauer \emph{et
al.}2021). Dams also alter riverine habitat to become more lacustrine,
resulting in sediment build up, aquatic plant growth, finer substrates,
and elimination of spawning substrate needed for benthic fish
reproduction (Ward \& Stanford 1983; Johnson \emph{et al.} 1995;
Kemp\emph{et al.} 2011; Keller \emph{et al.} 2021). These shifts are
consistent with our findings, suggesting improved flow rates, habitat
connectivity and quality of necessary spawning substrates for
pelagophils and benthic spawners following dam removal.

As observed with habitat guilds, heterogeneity in reproductive guilds
increased, shifting from single-guild dominance to an equitable
distribution of dominance across guilds. The number of unique niches
available within a stream is positively associated with habitat
diversity and complexity (Walrath \emph{et al.} 2016). Because the
current study found stream condition and dissolved oxygen improved in
response to dam removal, habitat complexity also may have improved,
driving equity in guild presence. Although substrate was not monitored
in this study, it is likely that a shift in substrate also occurred,
providing an essential component of reproduction for several guilds
(e.g., benthic spawners that adhere eggs to coarse substrate).

CONCLUSIONS

Dam removals are often approached with hesitance due to perceived losses
of some fish species and the potential for adverse environmental impacts
(Bednarek 2001; Hart\emph{et al.} 2002; Downs \emph{et al.} 2011;
Magilligan \emph{et al.}2017). However, this system experienced
immediate improvements in stream flow, dissolved oxygen levels, QHEI
scores, IBI scores, and fish abundance. Rather than decreases in some
functional groups, abundances of most increased following removal. The
increase in less abundant functional groups resulted in greater
equivalence across all functional groups, increasing the functional
diversity of the fish assemblage as a result. Although past studies
indicate that immediate ecological responses to dam removal can be
limited or negative (Cheng \& Granata 2007; Stanley \emph{et al.} 2007;
Dorobek \emph{et al.} 2015), improvements in the functional diversity,
overall abundance and habitat observed in our study following dam
removal show no potential downside of dam removal. In fact, two fishes
not previously found in Illinois have been discovered in these rivers
since removal, likely in response to improved conditions: the Tippecanoe
Darter (\emph{Nothonatus tippecanoe} ) and the Streamline Chub
(\emph{Erimystax dissimilis} ; Tiemann \emph{et al.} 2021). Despite the
immediate improvements, we recommend continued monitoring of these
systems to ensure sustained restoration and to improve our understanding
of long-term ecological responses to dam removal.

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Table . Results from ANOVAs examining impact of dam removal on QHEI,
IBI, and environmental metrics. Significant P-values are bolded.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& \emph{df} & \emph{MS} & \emph{F} & \emph{P}\tabularnewline
\midrule
\endhead
QHEI & & & &\tabularnewline
River & 1 & 0.0005 & s & 0.6479\tabularnewline
Location & 2 & 0.1030 & 47.18 &
\textbf{P\textless{}0.0001}\tabularnewline
Pre/Post & 1 & 0.0070 & 3.21 & 0.0791\tabularnewline
L x P & 2 & 0.0071 & 3.25 & \textbf{0.0466}\tabularnewline
error & 53 & 0.0022 & &\tabularnewline
IBI & & & &\tabularnewline
River & 1 & 0.0403 & 2.47 & 0.1202\tabularnewline
Location & 2 & 0.1205 & 7.4 & \textbf{0.0012}\tabularnewline
Pre/Post & 1 & 0.5345 & 32.83 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.0592 & 3.64 & \textbf{0.0313}\tabularnewline
error & 71 & 0.0163 & &\tabularnewline
Temperature (\selectlanguage{ngerman}°C) & Temperature (\selectlanguage{ngerman}°C) & & &\tabularnewline
River & 1 & 0.0184 & 2.4 & 0.1257\tabularnewline
Location & 2 & 0.0008 & 0.11 & 0.8951\tabularnewline
Pre/Post & 1 & 0.1077 & 14.09 & \textbf{0.0004}\tabularnewline
L x P & 2 & 0.0002 & 0.03 & 0.9681\tabularnewline
error & 71 & 0.0076 & &\tabularnewline
Dissolved Oxygen(mg/L) & Dissolved Oxygen(mg/L) & & &\tabularnewline
River & 1 & 0.0371 & 1.96 & 0.1658\tabularnewline
Location & 2 & 0.0001 & 0.01 & 0.9932\tabularnewline
Pre/Post & 1 & 0.3872 & 20.45 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.0015 & 0.08 & \textbf{0.9246}\tabularnewline
error & 71 & 0.0189 & &\tabularnewline
Flow (m/s) & & & &\tabularnewline
River & \textbf{1} & 0.0538 & 14.31 & \textbf{0.0004}\tabularnewline
Location & 2 & 0.0244 & 6.49 & \textbf{0.003}\tabularnewline
Pre/Post & 1 & 0.0106 & 2.81 & 0.0993\tabularnewline
L x P & 2 & 0.0005 & 0.15 & 0.8653\tabularnewline
error & 53 & 0.0038 & &\tabularnewline
\bottomrule
\end{longtable}

Table 2. Results of PERMANOVA's examining impacts of dam removal on
habitat and reproductive guild abundances. Significant P-Values are
bolded.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
& \emph{df} & \emph{Mean Square} & \emph{R\textsuperscript{2}} &
\emph{F} & \emph{P}\tabularnewline
\midrule
\endhead
Habitat Guilds & & & & &\tabularnewline
River & 1 & 0.7144 & 0.1394 & 15.2459 &
\textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 0.1687 & 0.0658 & 3.5989 & \textbf{0.0025}\tabularnewline
Pre/Post & 1 & 0.5959 & 0.1163 & 12.7175 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.0744 & 0.0291 & 1.5893 & 0.1424\tabularnewline
error & 71 & 0.0469 & 0.6494 & &\tabularnewline
Reproductive Guilds & Reproductive Guilds & & & &\tabularnewline
River & 1 & 0.4522 & 0.0980 & 11.1144 &
\textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 0.1720 & 0.0746 & 4.2288 & \textbf{0.0006}\tabularnewline
Pre/Post & 1 & 0.7722 & 0.1673 & 18.9795 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.0787 & 0.0341 & 1.9341 & 0.0720\tabularnewline
error & 71 & 0.0407 & 0.6260 & &\tabularnewline
\bottomrule
\end{longtable}

Table 3. ANOVA results assessing the impact of dam removal on habitat
and reproductive guild diversity (H'). Significant P-values are bolded.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& \emph{df} & \emph{Mean Square} & \emph{F} & \emph{P}\tabularnewline
\midrule
\endhead
Habitat Guild Diversity & Habitat Guild Diversity & & &\tabularnewline
River & 1 & 0.494 & 6.74 & \textbf{0.0115}\tabularnewline
Habitat & 2 & 0.615 & 8.40 & \textbf{0.0005}\tabularnewline
Pre/Post & 1 & 1.197 & 16.33 & \textbf{0.0001}\tabularnewline
L x P & 2 & 0.197 & 2.67 & 0.0750\tabularnewline
error & 71 & 0.073 & &\tabularnewline
Reproductive Guild Diversity & Reproductive Guild Diversity & &
&\tabularnewline
River & 1 & 0.004 & 0.06 & 0.8007\tabularnewline
Habitat & 2 & 0.426 & 7.10 & \textbf{0.0015}\tabularnewline
Pre/Post & 1 & 1.742 & 29.06 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.300 & 5.01 & \textbf{0.0092}\tabularnewline
error & 71 & 0.060 & &\tabularnewline
\bottomrule
\end{longtable}

Table 4. Results of ANOVAs examining impacts of dam removal on
abundances of each guild. Significant P-values are bolded.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& \emph{df} & \emph{MS} & \emph{F} & \emph{P}\tabularnewline
\midrule
\endhead
Habitat Generalist & & & &\tabularnewline
River & 1 & 5.21 & 27.77 & \textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 1.01 & 5.41 & \textbf{0.0065}\tabularnewline
Pre/Post & 1 & 6.38 & 34.01 & \textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.56 & 2.97 & 0.0578\tabularnewline
error & 71 & 0.19 & &\tabularnewline
Lobate Margin & & & &\tabularnewline
River & 1 & 18.67 & 46.02 & \textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 1.76 & 4.35 & \textbf{0.0166}\tabularnewline
Pre/Post & 1 & 1.77 & 4.36 & \textbf{0.0403}\tabularnewline
L x P & 2 & 1.01 & 2.48 & 0.0910\tabularnewline
error & 71 & 0.41 & &\tabularnewline
Riffle & & & &\tabularnewline
River & 1 & 7.32 & 15.56 & \textbf{0.0002}\tabularnewline
Location & 2 & 4.39 & 9.33 & \textbf{0.0003}\tabularnewline
Pre/Post & 1 & 12.73 & 27.06 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.97 & 2.07 & 0.1337\tabularnewline
error & 71 & 0.47 & &\tabularnewline
Run & & & &\tabularnewline
River & 1 & 0.01 & 0.00 & 0.9978\tabularnewline
Location & 2 & 2.06 & 6.44 & \textbf{0.0027}\tabularnewline
Pre/Post & 1 & 11.93 & 37.35 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.98 & 3.07 & 0.0525\tabularnewline
error & 71 & 0.32 & &\tabularnewline
Slackwater & & & &\tabularnewline
River & 1 & 3.44 & 14.81 & \textbf{0.0003}\tabularnewline
Location & 2 & 0.97 & 4.16 & \textbf{0.0197}\tabularnewline
Pre/Post & 1 & 0.38 & 1.62 & 0.2070\tabularnewline
L x P & 2 & 0.14 & 0.61 & 0.5474\tabularnewline
error & 71 & 0.23 & &\tabularnewline
Benthic Spawner & & & &\tabularnewline
River & 1 & 0.36 & 1.43 & 0.2365\tabularnewline
Location & 2 & 1.15 & 4.55 & \textbf{0.0138}\tabularnewline
Pre/Post & 1 & 5.48 & 21.71 & \textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 0.66 & 2.61 & 0.0808\tabularnewline
error & 71 & 0.25 & &\tabularnewline
Brood Hider & & & &\tabularnewline
River & 1 & 4.40 & 10.36 & \textbf{0.0019}\tabularnewline
Location & 2 & 3.99 & 9.38 & \textbf{0.0002}\tabularnewline
Pre/Post & 1 & 15.08 & 35.50 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 1.07 & 2.52 & 0.0874\tabularnewline
error & 71 & 0.43 & &\tabularnewline
Nest Builder & & & &\tabularnewline
River & 1 & 8.44 & 32.70 & \textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 0.50 & 1.93 & 0.1534\tabularnewline
Pre/Post & 1 & 0.48 & 1.86 & 0.1764\tabularnewline
L x P & 2 & 0.08 & 0.30 & 0.7394\tabularnewline
error & 71 & 0.26 & &\tabularnewline
Pelagophil & & & &\tabularnewline
River & 1 & 5.86 & 24.14 & \textbf{P\textless{}0.0001}\tabularnewline
Location & 2 & 1.32 & 5.42 & \textbf{0.0064}\tabularnewline
Pre/Post & 1 & 12.09 & 49.78 &
\textbf{P\textless{}0.0001}\tabularnewline
L x P & 2 & 1.13 & 4.64 & \textbf{0.0128}\tabularnewline
error & 71 & 0.24 & &\tabularnewline
\bottomrule
\end{longtable}

\emph{Figure 1. Map of sample sites on the Vermilion and North Fork
Rivers . Circles indicate sites on the Vermilion River, triangles
represent site on the North Fork River and the red bars indicate the dam
removal sites. DWN : Downriver of Dam, IMP: Impounded, ROR: Run of
River.}

\emph{Figure 2. Response of environmental parameters to river location
and dam removal. Grey indicate pre-removal values and white indicate
post-removal values. Values plotted are means +/- standard error. Refer
to Figure 1 for explanation of location abbreviations.}

\emph{Figure 3. Response of QHEI to river location and dam removal of
each habitat on both rivers. Grey markers are average pre-removal values
and white markers are average post-removal, +/- standard error. Dashed
lines are labeled to correspond score to the health of the river. Refer
to Figure 1 for explanation of location abbreviations.}

\emph{Figure 4. Response of Index of Biotic Integrity (IBI) to river
location and dam removal of each habitat on both rivers. Grey markers
are average pre-removal values and white markers are average
post-removal, +/- standard error. Dashed lines are labeled to correspond
score to the health of the river. Refer to Figure 1 for explanation of
location abbreviations.}

\emph{Figure 5. Response of total fish abundances to dam removal in each
river location. Grey bars indicate pre-removal values and white bars
indicate post-removal values. Values plotted are means +/- standard
error. Refer to Figure 1 for explanation of location abbreviations.}

\emph{Figure 6. Non-metric multidimensional scaling (NMDS) ordination
displaying impacts of dam removal on habitat and reproductive guild
abundances and habitat metrics. Guilds are displayed as black vectors
and QHEI and IBI are displayed as a grey vectors. Refer to Figure 1 for
explanation of location abbreviations.}

\emph{Figure 7. Response of guild diversity to dam removal. Grey bars
indicate pre-removal values and white bars indicate post-removal values.
Values plotted are means +/- standard error. Refer to Figure 1 for
explanation of location abbreviations.}

APPENDIX A: ADDITIONAL FIGURES

Figure A1. Response of habitat guild abundances to river location and
dam removal. Grey bars indicate pre-removal values and white bars
indicate post-removal values. Values plotted are means +/- standard
error. Refer to Figure 1 for explanation of location abbreviations.

Figure A2. Response of reproductive guild abundances to river location
and dam removal. Grey bars indicate pre-removal values and white bars
indicate post-removal values. Values plotted are means +/- standard
error. Refer to Figure 1 for explanation of location abbreviations.\selectlanguage{english}
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