Stability of riffle-pool morphology in Ozark streams

Ozarks streams have undergone extensive gravel aggradation over the last 50 to 100 years. Gravel aggradation can alter bed material characteristics, channel geometry, and channel pattern; these changes, in turn, can affect aquatic ecology, erosion rates, flood hazards, and water quality. In a comprehensive study of Ozarks streams, changes in channel cross-section geometry and channel pattern are being evaluated over time intervals of seasons to decades. Aerial photographs of Little Piney Creek (Phelps County, Missouri) from 1938 and 1989 show that the frequency and area of pools and riffles have changed substantially over the last five decades. In that time interval, numbers of pools, riffles, and runs (hydraulic units with a trapezoidal cross-section and no thalweg) have increased; the average sizes of pools and riffles, however, have decreased and the average size of runs has remained nearly constant. These changes have been accompanied by lateral channel migration of as much as 150 meters. Channel geometry changes and channel migration have been concentrated in discrete longitudinal zones characterized by riffles, mid-channel bars, and anastomosing channels; runs have been the most stable stream hydraulic units. Intensive channel cross-section monitoring of seven reaches of three streams (Little Piney Creek and Jacks Fork inmore » Missouri, and the Buffalo River in Arkansas) is being conducted to evaluate short-term changes in channel morphology. Comparisons between initial and resurveyed conditions over 1 to 3 years indicate highly variable changes in channel morphology.« less