<div>In field observations from a sinuous estuary, the drag coefficient C,
much greater than expected from bottom friction alone. C are explained
by form drag from flow separation at sharp channel bends. Greater water
depths during flood tides corresponded with increased values of
C<sub>D</sub>, consistent with the expected depth dependence for
flow separation, as flow separation becomes stronger in deeper water.
Additionally, the strength of the adverse pressure gradient downstream
of the bend apex, which is indicative of flow separation, correlated
with C<sub>D</sub> during flood tides. While C<sub>D</sub>
generally increased with water depth, C<sub>D</sub> decreased for
the highest water levels that corresponded with overbank flow. The
decrease in C<sub>D</sub> may be due to inhibition of flow
separation with flow over the vegetated marsh. The dependence of
C<sub>D</sub> during ebbs on discharge corresponds with inhibition
of flow separation by a favoring baroclinic pressure gradient that is
locally generated at the bend apex due to curvature-induced secondary
circulation. This effect increases with stratification, which increases
with discharge. Additional factors may contribute to the high drag,
including secondary circulation, multiple-scales of bedforms, and
shallow shoals, but the observations suggest that flow separation is the
primary source.</div>