EXAMINING THE SPATIAL VARIABILITY OF CHANNEL MORPHOLOGY AND PARTICLE SIZE OF FOUR MILE CREEK, OHIO

Channel morphology and particle size analysis are two key metrics used in the U.S. to assess the equilibrium state of a stream and determine if the stream needs to be restored. In general, a stream that is deeply incised with a channel that is cut off from its floodplain is considered to be out of equilibrium. We surveyed 8 channel cross-sections at roughly 2.5-km intervals along an 18-km reach of Four Mile Creek in southwestern Ohio to assess the variability in channel morphology. Wolman pebble counts were executed at 4 of these sites in order to determine the median (D₅₀) particle size of the stream. Bankfull discharge was estimated from locating the highest visible flotsam layers on the banks of each cross-section. The width of the flood-prone area was determined by measuring a height double the depth from bankfull discharge to the deepest point in the channel and measuring this elevation across the floodplain. Entrenchment ratios for each transect were determined by dividing the width of the flood-prone area by the bankfull discharge width. Low entrenchment ratios indicate a more entrenched channel. The results show that Four Mile Creek is more entrenched in the upstream portion of the study area as the entrenchment ratio increased from its lowest value of 1.12 at Site 1 (the most upstream site) to its highest value of 27.7 at Site 7. This trend indicates the stream is less connected to its floodplain in the upstream portion of the study area. Due to the predominance of agricultural land use in this watershed post-European settlement, it is possible that the stream is still adjusting to these land-use changes. Width to depth ratios (bankfull discharge width compared to maximum depth) also varied, with an initial decrease from its maximum value of nearly 30 at Site 1 to its minimum value of just over 11 at Site 3 and Site 4. A series of bedrock knickpoints is present between Site 4 and Site 5. The downstream half of the study area, which includes Site 5 through Site 8 below the knickpoints, has a relatively stable width to depth ratio of ~15. By comparing the changes in stream channel morphology and particle size throughout this reach, we can better identify the natural and human-induced controls that influence the current channel equilibrium of Four Mile Creek.