Geomorphic Evolution of a Gravel‐Bed River Under Sediment‐Starved Versus Sediment‐Rich Conditions: River Response to the World's Largest Dam Removal

Abstract

Understanding river response to sediment pulses is a fundamental problem in geomorphic process studies, with myriad implications for river management. However, because large sediment pulses are rare and usually unanticipated, they are seldom studied at field scale. We examine fluvial response to a massive (~20 Mt) sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States, in an 11‐year before‐after/control‐impact study of channel morphology and grain size. We test the hypothesis that for a given flow magnitude, greater geomorphic change occurs under sediment‐rich conditions than under sediment‐starved conditions. Channel response to flow forcing was significantly different during the sediment‐pulse peak, 1–2 years after dam removal began, than earlier or later. During peak sediment supply our hypothesis was supported; major geomorphic change occurred under low flows and unit stream power ≤60 W/m2. However, by 4–6 years after dam removal began, rates of geomorphic change and sensitivity to stream power had decreased substantially such that our hypothesis was no longer unequivocally supported. These findings are consistent with a two‐phase conceptual model of dam‐removal response, involving a transport‐limited state followed by a more supply‐limited state. From comparisons with other dam removals and natural sediment pulses, we infer that the longevity of sediment‐pulse signals in gravel‐bed rivers depends upon gradient, river discharge, valley morphology, and sediment grain size. Stream power associated with substantial geomorphic change varies with sediment supply, such that assigning a general threshold stream power to gravel‐bed rivers may be untenable.

A decadal‐scale numerical model for wandering, cobble‐bedded rivers subject to disturbance

“…East et al . () did find that reductions in upstream sediment supply had little effect on channel stability in the downstreammost reaches of the Elwha River (downstream of Elwha Dam) but suggest that sediment recruitment from large bluffs in that area may constitute a significant portion of the sediment budget. While we conclude that dam emplacement in systems similar to the Elwha River results in increased bed stability and a reduction in channel width, our model results imply that channel‐floodplain exchange does still occur in sediment‐starved systems, albeit less frequently and at a much lower magnitude.…”

“…() both found that the Middle Elwha was stable in the decade prior to dam removal; Pohl () concludes that the reach had low bed mobility compared to reaches upstream of the dam, and East et al . () did not observe channel width change in the reach following a 50‐year flood, but admit that data resolution is coarse in that area. East et al .…”

“…The model does not simulate the effect of in‐channel wood and is only appropriate for medium‐large streams where channel‐spanning log jams typically do not form (Eaton et al ., ). It should be noted that dam removals may increase the supply of wood; for example, an eight‐fold increase in wood was observed on the Elwha River following removal of Glines Canyon and Elwha Dams (East et al ., ). The importance of increased wood load following dam removal is still not well understood but should be accounted for in future studies.…”

Multidecadal Geomorphic Evolution of a Profoundly Disturbed Gravel Bed River System—A Complex, Nonlinear Response and Its Impact on Sediment Delivery

“…We have learned much by analyzing fluvial responses to past events. Studies have shown that geomorphic responses to fluvial disturbances (1) relax over varying timescales but can persist for years to decades, even centuries; (2) can be both acute and gradual; (3) are influenced by many factors including hillslope‐channel coupling, physical linkages among channel reaches, and hydrological regimes; (4) can follow many trajectories; (5) are intricately intertwined with ecological recovery; and (6) that systems may not return to predisturbance states (e.g., Bellmore et al, ; Cluer & Thorne, ; East et al, ; Foley et al, ; Gellis et al, ; Gran et al, , ; James, ; James & Lecce, ; Kasai, ; Kasai et al, ; Major et al, ; Moody & Meade, , ; Pierson & Major, ; Schumm, ; Swanson & Major, ). But we can further exploit records of physical responses to profound fluvial disturbances to gain richer geomorphic insights.…”

“…Analyses of geomorphic responses to fluvial perturbation have been approached in many ways. But aside from small‐scale analog models (e.g., Baynes et al, ; Lisle et al, ; Schumm, ; Schumm et al, ) and numerical models (e.g., Limaye & Lamb, ; Malatesta et al, ), few studies actually document long‐term channel evolution following disturbance of a dynamically stable river (e.g., East et al, ; Gellis et al, ; Gran et al, ; Kasai, ; Kasai et al, ; Lauer et al, ; Leopold, ; Meyer et al, , ; Moody et al, ; Moody & Meade, , ; Tunnicliffe et al, ), and even fewer record spatiotemporal variations of the relative efficacies of vertical versus lateral adjustments in the fluvial system (e.g., East et al, ; Meyer & Martinson, ; Moody & Meade, ). This is not surprising; few rivers exist where substantial (kilometers‐long) reaches are effectively reset and reestablished over human (decadal) timescales or where it has been possible to sustain multiyear, let alone multidecadal, monitoring.…”