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Title: Hydraulic and environmental effects of channel stabilization : Twentymile Creek, Mississippi
Authors: Shields, F. Douglas
Hoover, Jan Jeffrey 1954-
Nunnally, Nelson R.
Killgore, K. Jack
Schaefer, Thomas E.
Waller, Terry N.
Keywords: Stream channelization--Mississippi--Environmental aspects
Twentymile Creek (Miss.)--Channelization
Publisher: Environmental Laboratory (U.S.)
U.S. Army Engineer Waterways Experiment Station.
Series/Report no.: Technical Report;EL-90-14
Abstract: Twentymile Creek, located in northeast Mississippi, was straightened and enlarged about 1910, 1936-37, and in 1966. Extreme channel instability followed the 1966 modifications, and corrective measures (placement of bank protection and construction of three grade control structures (GCS)) were taken between 1982 and 1988. Hydraulic and environmental studies described herein were performed to determine effects of the corrective measures. Channel surveys and hydrologic data from Twentymile Creek were used to compare hydraulic conditions just before, just after, and 7 years after construction of grade control structures at RM 11.7 and RM 19.9 in late 1982. Grade control structures did not halt general bed degradation, but did promote local aggradation for about 1 mile upstream of each structure. The channel degraded and widened downstream of the GCS. Rip rap revetments were inspected in 1989 and were functioning properly. Environmental studies were structured to investigate recovery mechanisms suggested by previous workers. Other investigators have noted that other incised Mississippi streams have recovered stability by forming low-flow channels and vegetated longitudinal berms within the enlarged section. Low-flow channels have been suggested as features to ameliorate channel modification impacts on aquatic habitats. Habitat ·value of pools created by local scour below GCS placed in channelized streams has also been previously noted. A poorly defined low-flow channel was observed in Twentymile Creek upstream of RM 9.1, but was not evident on 1980 survey cross sections. Low-flow channel capacity was about 100 cfs, which is equaled or exceeded about 30 percent of the time. Woody vegetation cover on bank lines of selected reaches of Twentymile Creek and two reference streams (Big Brown and Mubby-Chiwapa Creeks) was mapped from aerial photos taken before (1981) and 3 years after (1985) GCS construction. The reference streams have had similar histories of modification and similar watershed land-use patterns. One GCS has been constructed on Big Brown; none have been constructed along Mubby-Chiwapa. Woody vegetation cover increased from 64.1 to 71.7 percent along Twentymile, but was relatively static along the other two streams, decreasing from 98.4 to 95.5 percent along Big Brown and increasing from 86.0 to 87.5 percent along Mubby-Chiwapa. Aquatic habitat diversity was quantified for selected reaches along Twentymile and Mubby-Chiwapa Creeks by measuring depth, velocity, cover, and bottom type at regularly spaced points during summer low flow and using results to compute a Shannon function index. Fish were sampled concurrently from the same reaches using seines. Higher levels of aquatic habitat diversity were observed below GCS relative to other reaches. Fish species diversity and richness were also higher below GCS. Thirty-nine fish species were found in Twentymile, but only 22 species were found in Mubby-Chiwapa. Fish species diversity and habitat diversity were only weakly correlated.
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