Land Use Based Analysis of Fluvial Hydraulic Geometry Differences in the Illinois River Watershed, Ozark Plateaus Province, Benton and Washington Counties, AR

The Illinois River Watershed originates on the Springfield Plateau in Arkansas on the north western flank of the Ozark Plateaus Physiographic region then flows into Oklahoma before turning south to its confluence with the Arkansas River. The watershed is of critical significance because of dramatic land use changes in the headwaters due to rapid population growth in northwest Arkansas. A mixture of forest and agricultural (open pasture and poultry houses) land use dominates the watershed, while urban areas are expanding. Little is understood about the geomorphology of headwater streams in the Ozark Plateaus region. This research defines the range of hydraulic geometry for headwater streams in the Illinois River watershed and assesses differences in these values across the three land use types. Nine second order stream watersheds with areas between 12 and 30 square kilometers were studied. Each watershed has greater than 50% of urban, agriculture, or forest land use. A geomorphic survey was conducted, encompassing longitudinal profiles, cross-sections, and bankfull pebble counts using the modified Wolman method. Percent gravel in the bed decreases from at least 70% in agricultural streams to less than 30% in urban streams; and the urban stream with the least riparian vegetation had the highest percent bedrock in the channel. These data suggest significant anthropogenic impacts on sinuosity in these urban watersheds. Forested watersheds have multi-thread meandering channels with some riffle/pool development. Agricultural and urban watersheds have similar hydraulic geometries, straight entrenched channels with minimal riffle/pool development and a meandering thalweg. Comparing agricultural and urban watersheds, anthropogenic influences on bank vegetation may be the principal control on hydraulic geometry values. Analysis of these differences will improve our understanding of natural and anthropogenic landscape processes in this rapidly changing watershed.