In northwestern Ohio, the impact of humans on natural systems extends back hundreds of years as manifested by complete reorganization of fluvial systems. This study addressed historical changes in the geomorphology the Ottawa River using vibracores, trenches, textural and geochemical analyses, and C-14 and blue light optically stimulated luminescence (blue OSL) dating.
The total section thickness from trenching and vibracoring is approximately 4.5-m. The oldest sediment in the cores is interbedded massive sands and silts >1.5-m thick. These deposits represent a point bar succession that is likely post-glacial in origin. Overlying the sand is 75-cm thick interval of peats and organic-rich carbonaceous muds interbedded with thin sand horizons. Several C-14 analyses from this interval have calibrated ages of 4889 +/- 178 YBP, 4731 +/- 306 YBP, and 4547 +/- 326 YBP. These deposits are interpreted as hydromorphic paleosols and overbank flood deposits that formed following the rise of Lake Erie.
An interval 33-cm to 68-cm thick overlies the paleosols and consists of alternating massive or cross-bedded sands interbedded with discontinuous silts. One blue OSL date from these sands produced an age of 231 +/- 15 YBP, which approaches the age of land clearance in northwest Ohio (early-1800s). This interval is interpreted as channel and channel margin deposits.
Further up section, a cross-bedded sand layer, dated at -4 +/- 5 YBP and -9 +/- 5 YBP, corresponds with a major flood that occurred in this region during 1959. Overlying this historic flood layer are approximately 1.6-m of silty floodplain deposits that accumulated during a period of rapid population growth and construction upstream of the study area. The implications are that the Ottawa River flowed through riparian wetlands and had low banks prior to land clearance, and that excessive sediment loads dating from the agricultural period and suburbanization boom are responsible for 1.6-m of vertical floodplain aggradation.
Following this period, the landscape was re-vegetated thereby decreasing the sediment discharge. As a result, the Ottawa River incised into previously stored sediment which produced the existing stream morphology of an entrenched channel flowing between terrace-like 2.3-m tall streambanks that are still inundated annually.