EXAMINING LITHOLOGIC AND BASE LEVEL CONTROLS ON THE SIZE AND SHAPE OF ORDOVICIAN KOPE FORMATION KNICKPOINTS IN OHIO RIVER TRIBUTARIES USING STRUCTURE-FROM-MOTION PHOTOGRAMMETRY

Knickpoints developed at resistant limestone layers are the primary locations of bedrock erosion in the tributaries to the Ohio River near Cincinnati. These sedimentary rocks erode mainly by the process of quarrying of joint blocks. Recent examination of stream systems in the Cincinnati area shows a variety of knickpoint shape and size, from ~10 cm to over 2 m in height and a range of “horseshoe” shapes in planform. Here, we ask if knickpoint morphology reflects local bedrock control, or if it is more related to the position of each knickpoint within the drainage network. The streams we are studying are small tributaries to Mill Creek and the Little Miami River near to where they join the Ohio River. The Ohio River, which was rearranged several times by North American ice sheets that terminated near Cincinnati, is incised ̴100 m here, which sets the base level for these smaller stream networks. The streams are carved into flat-lying Paleozoic stratigraphy, particularly the Ordovician Kope formation, which is primarily limestone with inter-bedded shale layers. The Kope Formation in this area dips 4-7 degrees to the north but has not undergone extensive deformation. A photographic technique called structure-from-motion was used develop 3-dimensional computer models of eleven knickpoints within the Kope Formation. A dense point cloud, 3D mesh, color orthographic image, and DEM (digital elevation model) were created for each knickpoint from ̴200 photographs using Agisoft Photoscan software. While model accuracy varied, the average scaling error was ̴10 cm. This error is low relative to the average knickpoint width of over 3 meters. We will present measurements of knickpoint longitudinal profiles and plan view morphometry extracted from these datasets using a Geographical Information System (GIS). Preliminary results indicate that proximity to base level, rather than local bedrock stratigraphy, appears to be the main control on knickpoint height in these stream systems.