SURFICIAL GEOLOGIC MAP OF THE FOUR MILE CREEK VALLEY, SOUTHWESTERN OHIO

Surficial geologic mapping of fluvial terraces can inform stream restoration design by identifying which terraces were active just prior to European settlement and thereby represent “natural” stream conditions. While pre-European settlement conditions may no longer be fully attainable due to irreversible changes in the watershed, past conditions can still be used to assess what restoration methods may be most successful. Four Mile Creek is a representative example of a stream within the formerly glaciated region of the Midwest. Mapping can therefore aid in understanding stream characteristics in this region prior to European settlement.

An 18-km stretch of Four Mile Creek was mapped at a 1:9,500 scale and accompanied by three valley cross-sections and a longitudinal profile. A relative elevation model of the Four Mile Creek Valley was created from high-resolution LiDAR data and served as a basemap over which historic maps and aerial imagery were then georeferenced to identify historic units. Stratigraphic sections were described at 31 river outcrops and 14 backhoe trench sites and 54 organic and shell samples were radiocarbon dated for age constraint. Map units of fluvial terraces were delineated primarily on relative elevation above river level (Qt1=highest, Qt7=lowest), and secondarily based on age (Late Pleistocene, Early Holocene, Late Holocene, or Historic).

The steepest gradient (0.0039) is in the middle portion of the mapping area, which is a knickzone where Four Mile Creek is incising into bedrock. The upper and lower portions of the mapping area are lower gradient alluvial reaches (0.0014-0.0026). For terraces at the same relative elevations, the age tends to be younger in the lower portion of the mapping area compared to the upper portion and this offset may span up to 11,500 years. Fluvial terraces active just prior to European settlement were Qt6 and Qt7, and additionally Qt5 in the lower portion of the mapping area. These terraces span up to ~0.3 miles in width. Two paleochannels are remnant of channel avulsion events where the channels jumped multiple times historically. These channel avulsions and terrace widths mapped within the last 5,000 years suggest that stream restorations with broad floodplain are needed to allow for natural channel migration and avulsions.