OSL GEOCHRONOLOGY REVEALS HIGH RATES OF FLOODPLAIN AGGRADATION DURING SUBURBANIZATION OF THE OTTAWA RIVER WATERSHED, NW OHIO, USA

In northwestern Ohio, the reorganization of fluvial systems can be interpreted to show the impact of humans on natural systems. This study used vibracores, trenches, textural and geochemical analyses, as well as ¹⁴C and blue light optically stimulated luminescence (blue OSL) dating to address historical changes in the geomorphology of the Ottawa River. The total vertical section from trenching and vibracoring is approximately 4.5-m thick. 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 a 75-cm thick interval of peats and organic-rich carbonaceous muds interbedded with thin sand horizons. Three ¹⁴C analyses from this interval have ages of 4900-4400 cal YBP. These deposits are interpreted as hydromorphic paleosols and overbank flood deposits that formed in the Great Black Swamp following the rise of Lake Erie to approximately modern lake levels. An interval 33-cm to 68-cm thick overlies the paleosols and consists of alternating discontinuous silts with massive or cross-bedded sands with a blue OSL age of 231 +/- 15 YBP. This interval is interpreted as channel and channel margin deposits formed during aggradation resulting from excessive sediment inputs coincident with land clearance in the region. Further up section, a cross-bedded sand layer dated at -4 +/- 5 YBP to -9 +/- 5 YBP corresponds with a major flood that occurred in this region during January 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 housing construction in outlying suburbs during the post-WWII expansion of the Toledo metropolitan area. The implications are that excessive sediment loads due to land clearance, agriculture, and suburbanization are responsible for 1.6-m of vertical floodplain aggradation. Most recently, the re-vegetation of housing developments coupled with urbanization of storm run-off resulted in a deficiency of sediment supply versus stream discharge. As a result, the Ottawa River is incising into previously stored sediment, producing the existing morphology of an entrenched channel flowing between 2.3-m tall fill terraces that are still episodically inundated.