The shortcomings of “passive” urban river restoration after low-head dam removal, Ottawa River (northwestern Ohio, USA)

Abstract

The concept of “passive” river restoration after dam removal is to allow the river to restore itself, within constraints such as localized bank erosion defense where infrastructure or property boundaries are at risk. This restoration strategy encounters difficulties in an urban environment where virtually the entire stream corridor is spatially constrained, and stream-bank protection is widely required. This raises the question of the meaning of river restoration in urbanized settings. In such cases, the sedimentary record can document paleohydrologic or paleogeomorphic evolution of the river system to better understand long-term response to the removal of the dam. Secor Dam was a low-head weir on the Ottawa River flowing through the City of Toledo, Ohio, and its outlying suburbs. The dam was constructed in 1928 and removed in 2007 to enhance aquatic ecosystems, improve water quality, and avoid liability concerns. Predam removal feasibility studies predicted the hydrological and sedimentological responses for the dam removal and determined that reservoir sediments were not significantly contaminated. Postdam removal studies included trenching, sediment coring, geochronology, and surveying. The buried, pre-1928 channel was located and showed that watershed urbanization resulted in channel armoring. Incision in the former reservoir exhumed a woody peat layer that was subsequently shown to be a presettlement hydromorphic paleosol currently buried beneath 1.7 m of legacy sediments, mostly deposited since ca. 1959. Today, the river flows through an incised channel between fill terraces composed of legacy sediments. Additional coring and survey work documented that the channel lateral migration rates averaged 0.32 m/yr over the past ∼80 yr, and that the meander wavelength is increasing in response to dam removal. Using sediment budget concepts, significant channel bank erosion and lateral channel migration should be expected until this river system reworks and removes accumulated legacy sediments currently in floodplain storage. In this dam removal project, “active” restoration practices, such as riparian wetland restoration, would have been more in accord with scientific understandings. That did not happen in this case because of disagreements among different constituencies and because of limitations of funding mechanisms.

Dam removal: Listening in

“…Besides potentially adding to materials being transported downstream and creating hazards, such structures, if exposed, could affect upstream/downstream connectivity and modify reservoir responses to dam removal [ Wildman and MacBroom , ; Harris and Evans , ; Zunka et al ., ; Magilligan et al ., ]. Specifically, knickpoints eroding through reservoir sediment may stall on structures, resistant soil horizons, or bedrock outcrops, thereby halting upstream propagation and local channel widening [ Evans et al ., ]. This may slow reservoir sediment erosion [ Wilcox et al ., ; Zunka et al ., ] and inhibit fish passage and navigation.…”

Channel Evolution of Sandy Reservoir Sediments Following Low-Head Dam Removal, Ottawa River, Northwestern Ohio, U.S.A.

“…This study also involved 8 vibracores and 32 push cores in channel and overbank facies which were evaluated for sediment properties, paleohydrology, and historical channel change [5] [21]. Bedload sampled were obtained with a Halley-Smith bedload sampler on 11 occasions prior to dam removal and 17 occasions after dam removal.…”

“…Other components of this study involved geochronology ( 14 C, OSL, and age dating anthropogenic materials) for paleohydrology [5]; HEC-RAS modeling to predict the hydrologic effects of dam removal [4]; use of the DREAM-1 model [24] to predict reservoir sediment erosion [21]; and geochemical analysis of the sediments to assess sediment contamination [4].…”

“…The public has become aware of the hazards of some of these dams following the spectacular failure of the IVEX dam in 1994 [2] [3] and also become aware of the estimate $300 million necessary to repair the most hazardous dams. In northern Ohio, dams have subsequently been removed to eliminate owner liability [4] [5], to improve water quality [6] [7], for habitat improvement [8] [9], or for improved recreational opportunities [10].…”

“…Between approximately RK 14 and RK 34, the modern Ottawa River channel is incised about 2-m beneath its floodplain surface, creating the appearance of fill terraces. Because these terraces are inundated at regular intervals, and because of geochronology data about the age of the terraces [5], the terraces are interpreted as anthropogenic in origin. Along the margin of the fill terraces are numerous examples of undercut banks, rotational slumps, toeof-slope deposits, small colluvial fans, small-scale avalanche or soil-fall deposits, and loss of riparian zone trees through undercutting, tree-lean, and tree-fall into the channel [5].…”