Abstract
Efforts to restore urban rivers require an understanding of human-influenced changes in channel substrates. This study uses three naturally-occurring oxbows in a 3.5 km reach of Swan Creek, flowing through the City of Toledo, Ohio (USA) to reconstruct historical changes in channel substrate. Human impacts in the watershed were: 1) land clearance for agriculture (peaking in 1900-1920) and for suburban housing tracts (peaking in 1945-1970), followed by 2) the post-1940 creation of more efficient urban run-off systems from streets, parking lots, housing developments, and shopping centers. Historical aerial photographs and maps from 1935, 1940, 1950, 1963, 1974, and 1994 were georeferenced using ground control points, input to ArcGIS, and have root mean square error (RMSE) ranging from 0.19 - 0.77 m (average RMSE = 0.47 ± 0.20 m) when compared to the 2006 digital ortho quarter-quadrangle (DOQQ) image used as the basis for comparison. Results showed that channel sinuosity continually increased from 1.88 (1935) to 1.99 (2006). Two oxbows probably formed in 1913, and the third formed in 1940. Sediment cores and trenches were used to recognize historical channel substrates. Age control was provided by 14C geochronology and labels on food packaging materials found in flood layers. Grain-size analysis of channel substrates shows a historical coarsening-upward trend: the largest clast size interval (f5) changes from +0.78f in pre-1935 channels, to －1.15f in pre-1940 channels, to －1.69f in the 2006 channel. These results indicate recent urban runoff created fluvial pavements and increasing channel mobility as the stream removes legacy sediment from intrabasinal sediment storage.
Highlights
River channels can shift location through lateral channel migration, through chute cut-offs, through neck cut-offs and through avulsion
Historical aerial photographs and maps from 1935, 1940, 1950, 1963, 1974, and 1994 were georeferenced using ground control points, input to ArcGIS, and have root mean square error (RMSE) ranging from 0.19 - 0.77 m when compared to the 2006 digital ortho quarter-quadrangle (DOQQ) image used as the basis for comparison
The channel substrates are overlain by finer-grained deposits deposited after the oxbow has been disconnected from the active channel
Summary
River channels can shift location through lateral channel migration (erosion at the cutbank and deposition on the point bar), through chute cut-offs (re-occupation of a former chute channel crossing the point bar, followed by abandonment of the previous channel), through neck cut-offs (intersection of meander loops, followed by abandonment of the previous channel) and through avulsion (levee breaching and crevasse splay evolution, followed by abandonment of the previous channel). The last three of these channel migration processes can produce oxbows or abandoned channels (this paper will use the terms synonymously). The lowest portion of abandoned channel fills consists of coarse-grained sediment (gravel, coarse-grained sand, and shell fragments), deposited by bedload transport through the active channel prior to the cut-off event. These former fluvial channel deposits represent the channel substrates that existed at the time of channel cut-off and abandonment. Ohio, which flows through the southern part of the Toledo metropolitan area (Figure 1(A)) This river is a tributary for the Maumee River, which flows into Lake Erie, one of the Laurentian Great Lakes of North America. The lowest terrace (T3) is only slightly elevated above the modern floodplain, and studies on similar features near the study area have shown T3 to be an anthropogenic terrace consisting of legacy sediment that formed due to excess sediment supply linked to land clearance in the region between about 1850-1970 [14] [16]
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