HISTORICAL CHANGES IN THE GEOMORPHOLOGY OF THE OTTAWA RIVER (NW OHIO, U.S.A.) DUE TO URBANIZATION AND LAND CLEARANCE

Recent dam removal on the Ottawa River exposed streambanks 2.3-m tall that were partially inundated by the upstream reservoir. A series of peats and rooted carbonaceous muds were exposed at the base of the exhumed channel bank. A study of the stratigraphy of the channel banks was undertaken with vibracores, trenches, ¹⁴C and OSL dating, textural and geochemical analyses. The composite section thickness from trenching and vibracoring is about 4.5 m. The oldest sediments in cores were fluvial laminated and cross-bedded sands >1.5 m thick where the proportion of carbonized wood fragments increase upward. Overlying the sands is an interval 75 cm thick consisting of a series of peats and carbonaceous muds with abundant woody materials and thin, interbedded sand layers. Several ¹⁴C analyses near the top of this interval have calibrated ages of 4280 ± 100 YBP and 4030 ± 150 YBP. These deposits are interpreted as a series of hydromorphic paleosols in one or a series of riparian wetlands. The paleosol range from about 10 cm below to about 50 cm above the mean daily stage height of the modern Ottawa River. Overlying the paleosols is an interval <25 cm thick that consists of alternating planar laminated, massive, or cross-bedded sands interbedded with carbonaceous muds, all containing abundant woody debris. Four OSL dates from these sands produce ages ranging from 1335 ± 80 YBP to 290 ± 15 YBP. This interval is interpreted as thin, overbank flood deposits in an adjacent riparian wetland. The upper limit of these sandy deposits approaches the initial age of land clearance in NW Ohio (late-1700s). Overlying the wetlands is a prominent cross-bedded sand layer about 25 cm thick with two OSL dates of 55 ± 5 YBP and 50 ± 5 YBP. These probably correspond to a major flood in 1959 in this region. There are up to 1.6 m of silty floodplain deposits overlying this historical flood layer. The implications are that the Ottawa River had low banks and flowed through riparian wetlands prior to land clearance, and that excessive sediment loads dating from the 1950s (post-WWII suburbanization of the drainage basin) is responsible for 1.6 m of vertical aggradation of the streambanks (average sedimentation rate of 3.2 cm/yr), producing the existing stream morphology of an entrenched channel flowing between terrace-like 2.3 m tall streambanks that are still inundated annually.