The shortcomings of “passive” urban river restoration after low-head dam removal, Ottawa River (northwestern Ohio, USA)<subtitle>What the sedimentary record can teach us</subtitle>

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 diffi culties 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 fl owing 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 signifi cantly 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 fl ows through an incised channel between fi ll 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, signifi cant channel Evans, J.E., Harris, N., and Webb, L.D., 2013, The shortcomings of “passive” urban river restoration after low-head dam removal, Ottawa River (northwestern Ohio, USA): What the sedimentary record can teach us, in De Graff, J.V., and Evans, J.E., eds., The Challenges of Dam Removal and River Restoration: Geological Society of America Reviews in Engineering Geology, v. XXI, p. 161–181, doi:10.1130/2013.4121(13). For permission to copy, contact editing@geosociety.org. © 2013 The Geological Society of America. All rights reserved. on April 17, 2013 reg.gsapubs.org Downloaded from