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...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 <sup>14</sup>C 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 (f<sub>5</sub>) 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.展开更多
文摘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 <sup>14</sup>C 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 (f<sub>5</sub>) 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.
