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Regional Curve Reports

Assessment of Morphological Conditions of Streams in the Five Major Physiographic Regions of Kentucky

Stream channel geometry is a product of complex watershed processes, direct modifications such as straightening or relocation, and changes to the watershed imposed by land-use activities such as development, livestock grazing, land clearing, and road construction. One of the most significant effects of these human activities has been the alluviation of valley bottoms and extensive channel incision that impairs habitat by increasing substrate mobility and reducing bed variability. Incision also leads to bank erosion, which increases delivery of fine-grained sediments to the channel. The deposition of these sediments is one of the primary causes of stream impairment in Kentucky. Because these and other similar impairments are so closely related to channel geometry, evaluation of geomorphic characteristics is important not only for assessing stream conditions but also for identifying stream impairment sources and practical mitigation strategies.

One increasingly common method for evaluating geomorphic characteristics is the comparison of measured bankfull channel geometry and flow to estimates from regional geomorphic curves. Based on the premise that similarities in landscape characteristics lead to measurable similarities in channel geometry and flow, regional curves describe values of bankfull flow and/or channel cross-sectional area, width, and depth that can generally be expected for streams of a given drainage area within a given physiographic region. While regional curves do not reflect the site-specific conditions that form each channel, they do reflect similarities in geology, land use, valley use, and other parameters that characterize the region. Thus, the estimates derived from regional curves can provide a reliable point of reference for assessing stream conditions, especially in those channels where bankfull indicators are unapparent or ambiguous: they facilitate estimations of degree of incision, relative bank stability, some channel pattern characteristics, and sediment transport capacity. They are also critical for evaluating the reasonableness of channel dimension and flow designs for stream restorations.

Site Selection

In order to develop regional curves for Kentucky streams, the University of Louisville Stream Institute conducted an extensive examination and collection of stream geomorphic characteristics in each of the five major physiographic regions of Kentucky: the Mississippi Embayment, Bluegrass, Eastern Kentucky Coal Field, Western Kentucky Coal Field, and the eastern rim of the Mississippian Plateau. Criteria used to identify stream channels suitable for regional curve data collection included a wide range of drainage basin areas within the physiographic region; active stream-flow gauges with long-term hydrological records, preferred in order to determine discharge at the identified bankfull stage; and as many streams as possible having a channel environment that was alluvial, showed no signs of ongoing rapid morphological change, and geomorphic characteristics suitable for surveying of bankfull indicators.

The suitability of the channel for surveying of bankfull indicators was determined based on evaluation of the floodplain and channel morphology upstream and downstream of the gauge. At a minimum, the reach had to have (1) cross-sectional geometry with unambiguous indicators of the bankfull level and evidence of at least one bank having been formed by deposition (2) channel geometry that was not controlled by a structure, and (3) a drainage area that differed by no more that 10 percent from the drainage area at the gauge station. The bankfull level was determined according to the definition of bankfull flow proposed by Dunne and Leopold (1978), who described it as the flow that completely fills the channel so that its surface is level with the active floodplain. The active floodplain is the flat depositional surface adjacent to the channel that is constructed by the present river in the present climate and is frequently inundated by the river (Dunne and Leopold 1978). Dunne and Leopold also reported an approximately 1.5‑year average return interval for bankfull flow; in the identification of the active floodplain of Bluegrass assessment reaches, however, no minimum or maximum bankfull return period was assumed.

The primary indicators used to identify the active or actively-forming floodplain were fine-grained depositional features (Dunne and Leopold 1978). The characteristics of these features varied depending on channel morphology. Many incised channels had multiple depositional surfaces—low, flat terraces that had to be distinguished from the active floodplain. In those channels, the primary indicator was a low depositional bench, and the bankfull level was identified as the point at which the slope transitioned between steep and horizontal. In cases where smaller, indistinct channels were forming within an incised channel, a primarily flat, vegetated bench was the most consistently observed depositional feature. Other incised channels lacked flat terraces; instead, the region between the valley flat and the channel was only a gently sloped incline. The bankfull level coincided with the top of bank below the valley flat and was identified as the point at which the slope transitioned between steep and more gradual. In streams that were not incised, the bankfull level coincided with the top of bank and valley flat.

Data Collection and Analysis

At each selected site, cross sections and longitudinal profiles were surveyed, and flow and bed sediment data were collected to compute bankfull parameters and to identify the channel type according to the Rosgen classification system. Ordinary least-squares regression was then used to relate bankfull flow and/or bankfull channel area, width, and depth to drainage area. The effects of geology, historical land-use, and current land use on sediment loads and channel evolution were also considered in stream assessment and in development of the curves.

Results

Regional curve reports were produced for streams in each of the five physiographic regions and for headwater streams of eastern Kentucky:

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