Channel response to a dam‐removal sediment pulse captured at high‐temporal resolution using routine gage data

Abstract

In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge ($7.8 km yr −1 ) and peak ($2.6 km yr −1 ) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre-pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later.The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment-transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process-factors largely independent of high flow.In contrast, persistent changes occurred to out-of-channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Serviceindicated flood stages being attained by 3-43% smaller discharges compared to earlier in the study period.This study captured a two-signal response from the sediment pulse: (1) short-to medium-term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer-term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales.

Influence of a Post-dam Sediment Pulse and Post-fire Debris Flows on Steelhead Spawning Gravel in the Carmel River, California

“…If the downstream two sites are improving, rather than showing natural variability, it reflects the exhumation of spawning gravel through attenuation of the downstream terminus of the sediment pulse. This behavior is also not typical of previously-described post-dam sediment pulses (Cashman et al, 2021). We acknowledge the difficulty in interpreting complex bed dynamics using single annual measurements at widely-spaced, fixed locations on the river.…”

Simulated Dynamics of Mixed Versus Uniform Grain Size Sediment Pulses in a Gravel‐Bedded River

“…These are the sediment-pulse volumes typically encountered by river managers. In the past few years, more data sets of larger sediment inputs, primarily from larger dam removals (Cashman et al, 2021;East et al, 2015East et al, , 2018Harrison et al, 2018;Major et al, 2012;Pace et al, 2017;Ritchie et al, 2018;Warrick et al, 2015) and modeling studies using these data sets (Cui et al, 2014(Cui et al, , 2019De Rego et al, 2020) have appeared in the literature. In the future, more directly testing an updated model against these data sets would provide a robust validation of large sediment-pulse movement in rivers.…”

“…• Channel incision may occur instead of aggradation when pulse volume is relatively small and pulse grain size is finer than bed • Intermediate grain size pulses have the largest downstream effects; finer sizes translate quickly and coarser sizes disperse slowly • A mixed-distribution pulse with smaller median grain size than the bed increases bed mobility more than a uniform-distribution pulse (Benda et al, 2004;Moody & Martin, 2004;Murphy et al, 2018Murphy et al, , 2019Sankey et al, 2017). Anthropogenic perturbations can also alter the frequency and magnitude of sediment supply, such as through dam removal (Cashman et al, 2021;Czuba et al, 2011;Dow et al, 2020;East et al, 2015East et al, , 2018Major et al, 2012;Ritchie et al, 2018) and gravel augmentation (Arnaud et al, 2017;Gaeuman et al, 2017;Welber et al, 2020). Once delivered into a river, a pulse of sediment moves downstream through some combination of translation, dispersion, and attrition of particles (An et al, 2017;Cashman et al, 2021;Cui & Parker, 2005;East et al, 2015East et al, , 2018Lisle, 2008;Lisle et al, 1997Lisle et al, , 2001Major et al, 2012;Ritchie et al, 2018;Sklar et al, 2009;Sutherland et al, 2002) or can be stored within the river and floodplain network (Benda & Dunne, 1997a;Cashman et al, 2021;.…”

“…Anthropogenic perturbations can also alter the frequency and magnitude of sediment supply, such as through dam removal (Cashman et al, 2021;Czuba et al, 2011;Dow et al, 2020;East et al, 2015East et al, , 2018Major et al, 2012;Ritchie et al, 2018) and gravel augmentation (Arnaud et al, 2017;Gaeuman et al, 2017;Welber et al, 2020). Once delivered into a river, a pulse of sediment moves downstream through some combination of translation, dispersion, and attrition of particles (An et al, 2017;Cashman et al, 2021;Cui & Parker, 2005;East et al, 2015East et al, , 2018Lisle, 2008;Lisle et al, 1997Lisle et al, , 2001Major et al, 2012;Ritchie et al, 2018;Sklar et al, 2009;Sutherland et al, 2002) or can be stored within the river and floodplain network (Benda & Dunne, 1997a;Cashman et al, 2021;.…”

“…Along with flume experiments and model simulations with simple geometry, recent field studies describe how sediment pulses, particularly following dam removal, evolve over long reaches in actual rivers with downstream variations in bed sediment grain size, slope, and width (Cashman et al, 2021;East et al, 2018East et al, , 2015Harrison et al, 2018;Major et al, 2012;Pace et al, 2017;Ritchie et al, 2018;Warrick et al, 2015). Following the removal of Simkins Dam on the Patapsco River in Maryland, Cashman et al (2021) found weeks to months of translation and dispersion of the sediment pulse within the channel, which aggraded the bed and affected local hydraulics. Dispersion and deposition on floodplains persisted for years.…”