KNICKPOINT STYLES ON TRIBUTARY STREAMS IN THE KENTUCKY RIVER VALLEY, CENTRAL KENTUCKY

The tributaries of the Kentucky River in central Kentucky are predominantly bedrock streams and exhibit knickpoints at a variety of scales and origins. The Old Kentucky River was a tributary of the ancient Teays River system. During the Early Pleistocene (~ 1.2 to 1.5 Ma), the Teays system was obstructed by an early glacial advance, destroying the Teays system and leading to the integration of the early Ohio River system. This reduced the distance to base level for Kentucky River water flowing toward the Gulf of Mexico by > 600 km. This resulted in rapid erosion of the Kentucky River system to > 75 m below its final Teays-related profile. Tributary streams in turn have eroded in response to the lowering of the Kentucky River base level. The bedrock geology of central Kentucky comprises gently dipping Paleozoic sedimentary rocks.

Knickpoints developed on individual resistant beds are the smallest scale of knickpoint recognized in this study. These beds tend to be a meter or less in scale and form small, discrete steps in the stream profile.

Certain resistant bedrock formations form larger deflections in the stream profile. These larger straigraphic knickpoints are similar in concept to the small resistant-bed knickpoints, and are essentially an agglomeration of the smaller knickpoints. They are larger scale and are usually more subtle in the field. They can, however, be detected in topographic profiles, and are commonly expressed as rapids or shallow reaches of the stream channel.

A third and different style of knickpoint results from dynamic geomorphic adjustment of the tributary stream profile to the incision of the main stem Kentucky River. These knickpoints are not obvious in the field, but are identifiable from longitudinal stream profiles. These knickpoints are not clearly related to lithologic variation or structures in the underlying bedrock. Instead, they show a systematic relationship between the distance of the knickpoint from the mouth of the tributary and the drainage area of the tributary system above the knickpoint. This suggests that the drainage area of the tributary is the primary control on the rate at which the stream has been able to adjust to the incision of the Kentucky River. Bedrock lithology plays a more limited role in the migration of the knickpoint through time.