Channel Stability of the Neosho River Downstream from John Redmond Dam, Kansas

By Kyle E. Juracek

Proceedings, International Conference on Riparian Ecology
and Management in Multi-Land Use Watersheds,
American Water Works Association,
Portland, Oregon,
August 28-31, 2000

The construction and operation of a reservoir can have a substantial effect on the stability of the river channel downstream from the dam. Since its completion in 1964, the downstream effect of John Redmond Dam on the Neosho River has been much debated. Anecdotal evidence of perceived channel widening has been provided by residents living along the river as far as 265 km downstream from the dam. Priority issues related to the stability of the Neosho River channel include protection of riparian resources, protection of habitat for threatened and endangered species (e.g., the Neosho Madtom, Noturus placidus), and bank stabilization as related to loss of property, general aesthetics, and recreation. To determine the possible effect of John Redmond Dam, a study of the Neosho River that compared the channel before, during, and after completion of the dam was undertaken by the U.S. Geological Survey (USGS) in cooperation with the Kansas Water Office.

The focus of this study was the 290-km reach located between John Redmond Dam and the Kansas-Oklahoma State line (see Figure 1). Throughout this reach the Neosho River is characterized by a meandering, gravel-bed channel that frequently is situated on bedrock. Channel slope averages about 0.2 m/km (Carswell and Hart, 1985). Riverbank height varies from about 4.5 to 9.0 m (U.S. Army Corps of Engineers, 1972). Alluvium in the Neosho River Valley averages about 7.5 m and is typified by silt with a basal layer of sand and gravel that averages about 1 m in thickness (Morton and Fader, 1972). The channel-bank materials consist mostly of cohesive silt and clay (Osterkamp and Hedman, 1981) and are relatively resistant to erosion compared to sand banks. Also, the channel banks are typically covered by partial to complete mature tree cover. Several tributaries contribute unregulated flow to the Neosho River downstream from John Redmond Dam. Changes in the streamflow regime attributable to the operation of John Redmond Dam have included a decrease in the magnitudes of peak discharges and an increase in the magnitudes of low discharges (Studley, 1996). Suspended-sediment concentrations are substantially reduced immediately downstream from the dam.

Pre- and post-dam channel stability was assessed using multiple-date aerial photographs. Five 10-km river reaches were selected for use in this study (Figure 1) with the objective being to obtain a spatially representative sample while avoiding, to the extent possible, localized human-caused or natural conditions that might obscure channel adjustment. The river reaches are located 42, 72, 106, 169, and 240 km downstream from the dam. Aerial photographs were obtained for three time periods--pre-dam (late 1930's), construction (early 1960's), and post-dam (early 1990's). For each reach, the bankfull channel area and channel centerline were interpreted from the aerial photographs for each time period and digitized. Primary indicators used in the delineation of the bankfull channel included breaks in slope, the tops of point bars, changes in vegetation, and shadows. Mean bankfull channel width was estimated as channel area divided by channel centerline length (Martinson, 1984). Pre-dam change was computed as the percentage difference in mean bankfull channel width between the pre-dam and construction time periods. Similarly, post-dam change was computed for each reach as the percentage difference in mean bankfull channel width between the construction and post-dam time periods.

Information from USGS streamflow-gaging stations at Burlington and Iola, Kansas (located 8 and 90 km downstream from the dam, respectively) (Figure 1) was also analyzed to assess pre- and post-dam channel stability. The comparison included an assessment of stage-discharge, discharge-width, discharge-area, and discharge-velocity relations for a range of in-channel flows. The gaging stations provided site- specific information at locations separate from the river reaches analyzed in this study.

Together, the aerial-photograph and gaging-station analyses indicate that the overall post-dam change in the Neosho River channel downstream from John Redmond Dam has been minor. With one exception (a 10-percent increase in width for the Iola reach), the five river reaches indicated little if any post-dam change in channel width. However, modest channel widening at the Burlington gage site, as well as at two of six U.S. Army Corps of Engineers' cross sections located within 10 km downstream from the dam, show that some localized widening has occurred. Localized widening may or may not be directly related to the operation of John Redmond Dam.

The overall lack of a pronounced post-dam channel change may be attributable to several factors. First, there has been a substantial post-dam reduction in the magnitude of the annual peak discharges. A second factor is the strength of the bed and bank materials. Degradation of the channel bed has been limited due to the presence of bedrock and (or) coarse gravel, the latter of which would require large flows to entrain and transport. The channel banks consist mostly of cohesive silt and clay and are relatively resistant to erosion. Moreover, bank stability may be enhanced at some locations by partial to complete mature tree cover. Therefore, significant bank erosion (beyond site-specific occurrences) may only occur during large flows, which essentially have been eliminated by the dam.

A third factor is the pre-dam condition of the channel. Because the LeRoy, Humboldt, and Oswego reaches had respective pre-dam increases in channel width of 15, 14, and 10 percent, there is some indication of a pre-dam widening of the channel, possibly due to a series of large floods that predated construction of the dam (including one with an estimated 500-year recurrence interval). It is possible that the channel, at least in places, may have been in an overwidened condition at the time of dam construction. Thus, additional widening at such locations may be unlikely.

The findings of this study suggest two possibilities. First, the Neosho River is a relatively stable system that may only change significantly in response to extreme events. Second, it may be that insufficient time has transpired for pronounced channel changes to become manifest. Local residents' perception of post-dam channel widening may be correct at specific locations. However, also likely are instances where normal channel migration has been mistaken for channel widening, especially where property (e.g., cropland, structures) has been lost or is threatened.

SELECTED REFERENCES

Carswell, W. J., and R. J. Hart, 1985. Transit Losses and Traveltimes for Reservoir Releases During Drought Conditions Along the Neosho River from Council Grove Lake to Iola, East-Central Kansas. U.S. Geological Survey Water-Resources Investigations Report 85-4003, 40 pp.

Martinson, H. A., 1984. Channel Changes of Powder River Between Moorhead and Broadus, Montana, 1939 to 1978. U.S. Geological Survey Water-Resources Investigations Report 83-4128, 62 pp.

Morton, R. B., and S. W. Fader, 1972. Ground Water in the Grand (Neosho) River Basin, Kansas and Oklahoma. U.S. Geological Survey Open-File Report 75-366, 35 pp.

Osterkamp, W. R., and E. R. Hedman, 1981. Channel Geometry of Regulated Streams in Kansas as Related to Mean Discharge, 1970-80. Kansas Water Office Technical Report No. 15, 56 pp.

Studley, S. E., 1996. Changes in High-Flow Frequency and Channel Geometry of the Neosho River Downstream From John Redmond Dam, Southeastern Kansas. U.S. Geological Survey Water-Resources Investigations Report 96-4243, 16 pp.

U.S. Army Corps of Engineers, 1972. Memorandum of Findings, Riverbank Stabilization Study, Grand (Neosho) River, Kansas & Oklahoma, John Redmond Dam & Reservoir to Kansas-Oklahoma State Line. U.S. Army Corps of Engineers, September 1972, Tulsa, Oklahoma, 24 pp.