THE RELATIONSHIPS BETWEEN MEAN BED LOAD DIAMETER, CHANNEL MORPHOLOGY, AND DRAINABE BASIN AREA AT KNICKPOINTS OF MILL CREEK, ILLINOIS

Many studies on knickpoints focus on stream system responses to base level drop, spatial variations in bedrock, tectonic events, or changes in discharge. However, in lower order streams, knickpoints often are associated with abrupt changes in sediment load or grain size distribution at the bed. Such changes are especially common near tributary junctions, where abrupt downstream increases in drainage basin area occur. It is commonly accepted that there is an overall decrease in grain size from the upper reaches of a channel profile to the lower reaches. A major cause for this decrease in grain size is an increase in discharge which allows a channel to decrease its slope and increase its width/depth ratio (change in width is typically more rapid than the change in depth). This study focuses on knickpoint morphology and its relation to variations in bed load grain size distribution, channel cross sectional area, and drainage basin area. The study site is Mill Creek, a tributary of the Rock River in Illinois. The goal of the study is to test the hypotheses that knickpoints are associated with tributary junctions and that at each knickpoint an abrupt decrease in mean bed load diameter occurs as the channel shape becomes wider and deeper downstream. Samples were collected from the three areas at knickpoint locations: (1) the knickpoint face, (2) the relict channel (channel unaltered by knickpoint formation), and (3) the adjusted channel (area immediately downstream of the knickpoint face). The grain size distribution and channel shape/area was taken for these three features of each knickpoint sampled. Results were then compared with other knickpoints based on their position on the channel profile and relative change in drainage area (size of tributary junction).