SEDIMENTOLOGICAL RESPONSE OF THE 2007 REMOVAL OF A LOW-HEAD DAM, OTTAWA RIVER, TOLEDO, OHIO

This project reports the transient effects of removal of a low-head dam on bedload sediment transport. The Secor dam (Ottawa River, Toledo, Ohio) was a run-of-river wier constructed in 1928 that was removed for liability reasons in a cooperative project involving several universities and state agencies. The former dam was 2-m tall and 17-m wide, and the former reservoir represented minor widening of the river for about 150 m upstream. The dam had limited trapping efficiency, thus the reservoir sediments were primarily sands (alternating layers of clean quartz sands and organic silty sands) with minor amounts of mud. Gravel-shell fluvial pavements were common in the upstream reaches. Removal of the dam on November 19, 2007 initiated rapid nickpoint migration to approximately 52-meters upstream within 12-hours. At this point, the nickpoint was incising through the sandy deposits of the former delta, and stabilized its position for several days, due to slower incision rates through an exhumed resistant layer (a silty sand layer with significant amounts of large woody materials). Incision of the former sandy delta caused mobilization of bedload that moved downstream as a sediment wave that could be visually tracked with GPS. During the first 24-hours, the sediment wave moved downstream at approximately 0.5 m/hr, but this slowed to an average of 0.13 m/hr when the sediment wave began to infill the former reservoir pool. Meanwhile, gravel from fluvial pavements directly upstream of the former spillway formed a pebble-sized gravel bar immediately downstream of the former spillway.