Low-head dams can be built in ephemeral streambeds to trap sediments which can store water or serve as sand reserves for other uses. For sand dams to provide sustainable and dependable water supplies, or to provide valuable sand for other purposes, these reservoirs should primarily fill with coarse sand rather than fine sediments. The problem of sand dams being negatively impacted by an excess of fine sediments is a widespread issue. In Kenya, 40-60 percent of sand dams are reported to be affected by this problem, which can limit their ability to recharge and provide recoverable water. We describe a novel approach to preventing collection of fine sediments by geomorphic management of reservoir sedimentation. Specifically, we suggest building dams with “Eiffel Tower” shaped outlets (broad at the base and narrowing with height) to remain open until the reservoir is sediment filled. The opening is designed to provide constant Rouse number of 2.5 for 0.125 mm grains so that regardless of flow, only sand of size greater than 0.125 mm will accumulate. Considering the limitations of 1-dimensional simulations in capturing edge effects, a stage discharge relationship acquired through HEC-RAS simulation is utilized to correct the opening. Numerical modeling confirmed that these outlets maintain constant bed shear stress, and thus promote the deposition of uniform coarse sediments within the reservoir regardless of riverine flow rate. The findings of the HEC-RAS simulation demonstrate that bottom-notch openings, especially those of the “Eiffel Tower” shape, exhibit superior performance with an MSE value of less than 1% when determining the deviation between the desired Rouse number (2.5) and the calculated Rouse number.

Several bills moving through Congress are likely to provide significant funding for expanding research and results in climate change solutions (CCS). This is also a priority of the Biden-Harris Administration. The National Science Foundation (NSF) will be expected to distribute and manage much of this funding through its grant processes. Effective solutions require both a continuation and expansion of research on climate change–to understand and thus plan for potential impacts locally to globally and to continually assess solutions against a changing climate–and rapid adoption and implementation of this science with society at all levels. NSF asked AGU to convene its community to help provide guidance and recommendations for enabling significant and impactful CCS outcomes by 1 June. AGU was asked in particular to address the following: 1. Identify the biggest, more important interdisciplinary/convergent challenges in climate change that can be addressed in the next 2 to 3 years 2. Create 2-year and 3-year roadmaps to address the identified challenges. Indicate partnerships required to deliver on the promise. 3. Provide ideas on the creation of an aggressive outreach/communications plan to inform the public and decision makers on the critical importance of geoscience. 4. Identify information, training, and other resources needed to embed a culture of innovation, entrepreneurialism, and translational research in the geosciences. Given the short time frame for this report, AGU reached out to key leaders, including Council members, members of several committees, journal editors, early career scientists, and also included additional stakeholders from sectors relevant to CCS, including community leaders, planners and architects, business leaders, NGO representatives, and others. Participants were provided a form to submit ideas, and also invited to two workshops. The first was aimed at ideation around broad efforts and activities needed for impactful CCS; the second was aimed at in depth development of several broad efforts at scale. Overall, about 125 people participated; 78 responded to the survey, 82 attended the first workshop, and 28 attended the more-focused second workshop (see contributor list). This report provides a high-level summary of these inputs and recommendations, focusing on guiding principles and several ideas that received broader support at the workshops and post-workshop review. These guiding principles and ideas cover a range of activities and were viewed as having high importance for realizing impactful CCS at the scale of funding anticipated. These cover the major areas of the charge, including research and solutions, education, communication, and training. The participants and full list of ideas and suggestions are provided as an appendix. Many contributed directly to this report; the listed authors are the steering committee.

The thermal regime of rivers plays a key role in aquatic ecosystem health. In the Willamette River, OR, main channel temperatures can be too warm for cold water fishes, causing fish to concentrate in secondary channel features including side channels, ponds, and alcoves. However, temperature regimes vary among and within features. Improved understanding of physical processes controlling thermal regimes in gravel-bed rivers is needed for targeted conservation action. This study characterized thermal regimes on the Willamette through field observations of temperature continuously measured at one side channel, eight alcoves, and six beaver ponds over a two month period. Insight into these measurements was provided by two dimensionless quantities. The Richardson number, characterizing stratification, was calculated with temperature and flow data. Values showed two well-mixed sites and 13 stratified sites. Stratification allowed calculation of the hyporheic-insolation number, characterizing the ratio of cooling flux from hyporheic discharge to heat transfer from incoming solar radiation. As calculated hyporheic-insolation numbers for sites increased, measured temperatures at sites decreased, showing a bin-averaged logarithmic fit R2=0.97. Results further indicate secondary channel features that provide cold water habitat are characterized by stratification and cool hyporheic discharge. Stratification is a necessary yet insufficient condition for cold water to provide habitat for aquatic biota because cold areas may still be anoxic, as suggested by dissolved oxygen point measurements. The hyporheic-insolation number has the ability to predict and thereby classify the thermal regimes of secondary channel features based on minimal field measurements and could guide floodplain restoration efforts.

Sand dams, a water harvesting system built in arid or semi-arid regions, collect and store water in saturated sands to increase water availability in dry seasons, while avoiding evaporation and reducing water-borne disease vectors. The capacity of the dam to store water depends on the texture of the sediment accumulated in the reservoir. An ideal sand dam is expected to wash fine-grained particles, especially silt and clay out of the reservoir, collecting only coarse particles to provide for maximum open pore space and minimum capillary retention (the water is typically extracted via an open well). Although conceptually simple, sand dam commonly failed due to the retention of fine particles. It has been recommended to build sand dams in stages to overcome this problem, with each stage low enough so that the shear forces of flow will keep silt and clay mobile, and pass them out of the reservoir. Although it is effective, this method is not preferred in terms of cost and time spent (repeatedly re-mobilizing a team to add to the dam). We present a new approach to the siltation problem by seeking an ideal shape (weir) cut in the face of a sand dam designed to provide shear forces such that coarse particles accumulation while washing the finer materials such as silt and clay. This will be done by finding a relation between the cut-outs of in the face of the dam and the sediment transport rates. Challenges in predicting sediment transport rate are investigated using both numerical and experimental modeling. We seek to reduce the failure rate of sand dams, and also provide for a method to re-establish sediment fills behind existing dams.

GeoHealth represents the critical intersection between the Earth and environmental sciences, and agricultural and health sciences. Following a specific request from the National Science Foundation (NSF) this report provides a series of recommendations aimed at empowering research, building fundamental workforce capacity, and improving communication around GeoHealth to the public and policy makers. This development is critical as a robust GeoHealth research enterprise is essential to global health, human and ecosystem well-being, and sustainability. The AGU community along with those from several allied societies provided the recommendations in this report; these were developed for a detailed survey and two workshops. The survey and other input revealed several broad challenges and needs, including highly siloed funding and support for researchers across institutions and societies, the inability to access or combine key datasets, and in particular the lack of clear career trajectories and support. The recommendations consist of: (i) six programmatic areas where significant attention to building a GeoHealth research enterprise is needed; (ii) approaches and concepts for four specific challenges in GeoHealth for which significant results could be enabled rapidly, within 2-3 years; (iii) ideas for developing an education/career path and for outreach; (iv) larger “moonshot” ideas that might yield very significant impacts over ca. 10 years. All of these have several common elements and themes: they leverage many directorates within NSF, including all within the GEO division; can build off of existing initiatives; are best developed through partnerships with other agencies and communities; and rely on open and FAIR data sets. Although the focus of these recommendations is toward and for the NSF, the suggestions are more general and hopefully will be considered by other funding agencies and other parts of the research enterprise in the U.S. and internationally.

Predicting the impact of changing climate and anthropogenic influences on stream discharge dynamics and baseflow conditions requires insight into the main factors that regulate storage and transfer of water from hillslope aquifers to surface streams. Classically, it is assumed that above a certain scale, hydrological laws involved at small-scale can be simplified, allowing the representation of the landscape and its subsurface in models as a homogeneous hillslope with effective slope, length and hydraulic properties. From a comprehensive analysis of hydrological, geological and geomorphological databases available in the Swiss Alps we provide evidence that such simplification might lead to inaccurate estimates of streamflow dynamics at baseflow. We reveal that recession behavior strongly deviates from that predicted by idealized homogeneous theories. A correlation analysis allows us to identify which key features of the landscape might control this deviation, with particular attention to slope, drainage density, depth to bedrock, and lithology as the main drivers. We summarize the current knowledge of physical mechanisms that could lead to complex hydrological behavior in Alpine contexts, and we finally discuss implications in defining modeling strategies for the Critical Zone community.