EVOLUTION OF THE OHIO RIVER VALLEY AFTER OUTWASH CEASED

Steep, relatively narrow, bedrock-controlled bottoms characterize the upper Ohio valley (above the falls at Louisville). These were filled with extensive outwash, slackwater, and other glaciofluvial deposits during the late Wisconsin, essentially choking the valley with coarse-grained sediment. Evidence from locales in Indiana, Kentucky, Ohio, and West Virginia show that after glacial discharge ceased (~14 ka), the upper Ohio River underwent several millennia of down-cutting and lateral erosion to “clean out” these sediments. Significant readjustment of this pattern, however, occurred 7-4 ka when vertical accretion/overbanking and levee construction dominated. Since 3 ka (except for fans at the mouths of major tributaries) the upper Ohio River has been confined mainly to its existing channel belt with only limited vertical accretion. This pattern suggests that initial, early Holocene erosion focused on cleaning outwash from the valley. The river style adjustment to vertical accretion and levee formation during the mid-Holocene (7-4 ka) may have been a response to midcontinental climate changes related to the final retreat of the Laurentide Ice Sheet from eastern North America after the Hypsithermal.

Conversely, the lower Ohio valley (below the falls) is dominated by broad, low glaciofluvial and floodplain bottoms. The stratigraphy of sediments within floodplain lakes such as Hovey (mouth of the Wabash) and Avery Lake (Black Bottom) indicate that channel incision dominated in the lower Ohio valley during the mid-Holocene. Deep (10-12 m), relatively narrow channels (compared to today) apparently characterized these locations until about 4-5 ka when they were abandoned and begin to infill with mud. The narrower and deeper morphology of mid-Holocene channels than observed historically (1-5 m) suggest that formery, channels rapidly avulsed rather than scrolled across the valley, which is comparable to channel processes for bottoms in the middle Mississippi valley. The channels must have avulsed far enough apart not to become filled with coarse grained sediment. If the locales at Hovey and Avery lakes are typical, then a different river system may have existed in the lower Ohio valley than is characteristic during historic time.