Catherine Noll (Geology) Oberlin College
Channel incision occurs as increased runoff from urbanization carries an above average sediment load that in turn grinds at the stream banks and bed, cutting it steeply. The water table then flows into the streambed, dropping as it fills the larger, deeper incised channel. At Upper Chisel Run, this has resulted in a knickpoint, above which the unincised channel maintains its natural water level and morphology, and below which the incised channel begins to re-equilibrate to the new runoff cycle.
The upstream, unincised portion of the channel was used as the basis of comparison for channel stability. Erosion pins were installed with a known portion exposed and measured for erosion and deposition. Transect A, the farthest upstream and least impacted by the incision, had little variation in shape and sediment erosion and deposition both on the banks and within the channel. This is in contrast to transect K, just upstream from the knickpoint, which eroded consistently in the stream channel and some in the bank. Little post-storm recovery was noted here. Just downstream from the knickpoint at transect C, deposition occurs as the water pools and unidirectional energy dissipates. Further downstream around transect D, deposition occurs around the banks, but the channel remains unstable, with large variation in the sediment load. Transect F is the most stable; here, a new "floodplain" has deposited within the widely eroded banks of what was once its floodplain.
By measuring the knickpoint retreat and taking channel cross-section measurements, the rate of channel morphology can be found. Based on the knickpoint movement from this summer and its position four years ago, it is concluded that it erodes at a rate of roughly 1.6 meters per year. Assuming a steady rate of movement, this would put its beginning about 10 meters beyond transect E, 88 meters from where it is now, since its supposed start in 1950. The banks of the channel undergo a different rate of movement. Upstream, around transect A, the banks are stable at about 3 meters across, with the channel about 30 cm deep. As the knickpoint retreats it widens and deepens slightly. Once it passes, there is an immediate bank retreat of about one meter and stream depth change to about 1.5 meters. Downstream of this the banks continue to widen at a rate of about one meter every 25 years, significantly slower than the knickpoint's retreat rate. At transect F, where the floodplain is rebuilding, the banks are about 8 meters apart and the stream channel shallows slightly to about 1.3 meters. However, it remains somewhat unstable (relative to transect A) and not entirely recovered as is apparent by the high variablity of erosion and depostion and the coarse and sandy floodplain relative to transect A's finer, organic floodplain. The implications of this study are that the geomorphic recovery following channel incision requires a timescale of over 50 years. Full recovery of the floodplain and natural vegetation will take even longer, thus it is important to recognize the impact of urbanization on wetland habitats early so measures can be taken to preserve such environments before they are destroyed.
For additional documentation Catherine Noll provided a PowerPoint Presentation entitled "Geomorphology along Upper Chisel Run, an incised channel" provided here in PDF form.