Are Streams in the Southwestern Ozarks Underfit? Applying Dury's model to the Illinois River Watershed in Arkansas.

George H. Dury (1964) proposed eight theoretical combinations of stream pattern and valley pattern that represent underfit streams; claiming underfit is a climate induced condition caused by a significant decrease in channel forming discharge. One combination was defined by the Osage River in the northeastern Ozark Plateaus of Missouri. Osage underfit streams fail to meander within a meandering valley. The mean channel meander wavelength and channel width of the stream is much less than the valley resulting in valley-stream ratios of up to 40:1 in contrast to his expected values of approximately 11:1. Dury's model is generally applied to the entire Ozarks including the Illinois River watershed without field data support. The Illinois River is located on the western flank of the Ozark Plateaus physiographic region on the Springfield Plateau which has different lithology than the Salem Plateau where the Osage River is located. To test the assumption that streams in the Illinois River watershed are underfit a combination of field, map, and GIS data were collected. Geomorphic surveys of ten reaches along eight first order streams were completed. The average stream widths of the ten reaches were compared to valley widths measured from USGS 1:24000 Quadrangle maps. The valley to stream ratios ranged from 1 to 15. Forested watersheds exhibited the highest width ratios, ranging from 12 to 15, while ratios in urban and agricultural watersheds were less than 2. This finding is consistent with observed changes in stream morphology caused by anthropogenic influences. To extrapolate to the larger watershed thirteen valley and stream widths along the Illinois River and two higher order tributaries, Osage Creek and Clear Creek, were measured from USGS maps. These ratios ranged from 2.8 to 5.7. Additionally, stream and valley wavelengths are being analyzed in a GIS using the USGS medium resolution hydrology data set and a LiDAR derived 8 m DEM for the watershed. These data suggest it is invalid to apply the Osage underfit model to this watershed. These findings are being validated with additional stream and valley width measurements in the field along the first order streams, Osage Creek, Clear Creek, and the Illinois River.