GRAVEL DISTRIBUTION AND LAND USE IN TWO HEADWATER STREAMS OF THE ILLINOIS RIVER WATERSHED, ARKANSAS

The Illinois River Watershed, in Northwest Arkansas, is of critical interest regionally because of its ecological, economical, and social value. Also of interest are impacts within the watershed resulting from extensive changes in land use in the headwaters. Current research on first order streams in the watershed has identified differences in bed materials tied to land use. First order streams in forested and agricultural areas have a gravel mantled bedrock base. In contrast, urban streams have significant areas of exposed bedrock, generally at the riffle crest. In order to elucidate this relationship eight cross-sectional pebble counts were conducted along two urban headwater streams. In both streams bedrock was exposed at multiple riffle crests, but not in a predictable pattern from upstream to downstream. Where bedrock was exposed it ranged from 11% to 72% of the bed material. The majority of the gravel was 11 mm to 256 mm in size (medium gravel to cobbles on the Wentworth Scale). The eight samples are statistically significant with a Pearson’s Chi Square probability of less than 0.0001, supporting the conclusion that this bed material distribution is tied to the watershed land use. The exposed bedrock and the associated lack of gravel have consequences for the biological and ecological health of the stream and must be considered in future restoration and protection projects within the watershed.