Flood Effects Provide Evidence of an Alternate Stable State from Dam Management on the Upper Missouri River

Abstract

We examine how historic flooding in 2011 affected the geomorphic adjustments created by dam regulation along the approximately 120 km free flowing reach of the Upper Missouri River bounded upstream by the Garrison Dam (1953) and downstream by Lake Oahe Reservoir (1959) near the City of Bismarck, ND, USA. The largest flood since dam regulation occurred in 2011. Flood releases from the Garrison Dam began in May 2011 and lasted until October, peaking with a flow of more than 4200 m 3 s À1 . Channel cross-section data and aerial imagery before and after the flood were compared with historic rates of channel change to assess the relative impact of the flood on the river morphology. Results indicate that the 2011 flood maintained trends in island area with the loss of islands in the reach just below the dam and an increase in island area downstream. Channel capacity changes varied along the Garrison Segment as a result of the flood. The thalweg, which has been stable since the mid-1970s, did not migrate. And channel morphology, as defined by a newly developed shoaling metric, which quantifies the degree of channel braiding, indicates significant longitudinal variability in response to the flood. These results show that the 2011 flood exacerbates some geomorphic trends caused by the dam while reversing others. We conclude that the presence of dams has created an alternate geomorphic and related ecological stable state, which does not revert towards pre-dam conditions in response to the flood of record. This suggests that management of sediment transport dynamics as well as flow modification is necessary to restore the Garrison Segment of the Upper Missouri River towards pre-dam conditions and help create or maintain habitat for endangered species. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Large wood distribution, mobility, and recruitment in an inter‐dam river reach: A comparison with geomorphic process on the Garrison Reach of the Missouri River pre and post the historical 2011 flood

“…The previously described 'dam proximal' reach matches nearly exactly with two erosional segments identified by the change point break for cumulative bank erosion. Skalak et al, 2013Skalak et al, , 2017Volke et al, 2015). The 'river dominated' reach, a section of river that should act similar to a free flowing river, matched well with three segments that showed mostly bank accretion during the flood.…”

“…As such this was one of the first studies of an inter-dam large river reach utilizing multiple LW surveys and channel change observations from both pre-flood and directly related to the flood (Angradi et al, 2004;Skalak et al, 2013Skalak et al, , 2017. As such this was one of the first studies of an inter-dam large river reach utilizing multiple LW surveys and channel change observations from both pre-flood and directly related to the flood (Angradi et al, 2004;Skalak et al, 2013Skalak et al, , 2017.…”

“…The discharge below the Garrison Dam in North Dakota exceeded 4390 m 3 /s for more than two weeks with high discharges continuing through the summer . The 2011 flood moved considerable sediment, generally scouring the main channel while depositing sediment and LW on sand bars and floodplains (Skalak et al, 2013(Skalak et al, , 2017Schenk et al, 2014b). For comparison, peak discharge of the next largest recorded flood during dam regulation (1975) was approximately one-half the discharge of the flood of 2011 (1951 m 3 /s at the USGS streamgage 06342500).…”

“…Understanding the unique geomorphic, ecological, and hydrologic attributes of inter-dam river reaches has become a priority as society begins to understand that these novel landforms have broad implications for ecosystem, flood, and navigation management (Johnson, 2002;Graf, 2006;Johnson et al, 2012;Skalak et al, 2013;Dixon et al, 2015;Volke et al, 2015). The relatively free flowing river reaches in between the dams and associated reservoirs have responded to paired upstream and downstream perturbations (Skalak et al, 2013(Skalak et al, , 2017Volke et al, 2015) by widening and deepening near the upstream dam and filling, braiding, and losing competence to transport the upstream load through downstream reaches (Stevens, 1938;Borland and Miller, 1960). The relatively free flowing river reaches in between the dams and associated reservoirs have responded to paired upstream and downstream perturbations (Skalak et al, 2013(Skalak et al, , 2017Volke et al, 2015) by widening and deepening near the upstream dam and filling, braiding, and losing competence to transport the upstream load through downstream reaches (Stevens, 1938;Borland and Miller, 1960).…”

“…A secondary goal of this study is to compare LW longitudinal patterns with the previous river segmentation to determine if LW longitudinal distributions mirror geomorphic trends (e.g. Skalak et al, 2017). Documentation of these longitudinal patterns in LW may serve as a proxy for understanding geomorphic river reaches (e.g.…”