U.S. Geological Survey
Water-Resources Investigations Report 99-4147
Geomorphic Effects of Overflow Dams on the Lower Neosho River, Kansas
By Kyle E. Juracek
Prepared in cooperation with the
KANSAS WATER OFFICE
Table of Contents
Figure 1. Overflow dam at Neosho Falls, Kansas.
Figure 2. Location of Neosho River Basin,
study area, and overflow dams.
Figure 3.Overflow dam hydraulics and geomorphic effects.
4. Leaf-off aerial photograph of the lower Neosho River channel near Burlington,
Kansas, December 1990.
Figure 5. Shapes of the Neosho
River channel, human-caused and natural conditions, and evidence of channel widening
downstream from the 12 overflow dam sites.
Figure 6. Erosion
on left bank of Neosho River just downstream from riprap at Oswego overflow dam.
Figure 7. Channel change within the
The purpose of this report is to characterize the geomorphic (channel-changing) effects of
overflow dams on the lower Neosho River channel in southeastern Kansas. Specifically, the
report describes the types, upstream and downstream extents, and stability of the geomorphic
effects in relation to site-specific, human-caused and natural conditions that may affect the
channel's response to the overflow dams.
Along the lower Neosho River in southeastern Kansas, a series of 12 concrete overflow
(low-water) dams have been constructed that extend across the full width of the river channel
(figs. 1 and 2). Most of the overflow dams
were built for water-supply purposes to serve nearby towns. The present-day (1999) overflow
dams were constructed mostly in the 1930's or 1950's. However, some of the dams were predated
by earlier versions. For example, at Humboldt the original overflow dam was a rock structure
built in 1875 in association with a mill. Aerial photographs from 193738 indicate that
earlier versions of dams also existed at Erie (just downstream from the present dam), Oswego,
and Chetopa (fig. 2). Typically, the overflow dams range
in height from about 3 to 6 feet above the low-flow water surface. An exception is the dam at
Burlington, which has a height of about 15 feet.
An understanding of the geomorphic effects of overflow dams on the Neosho River channel is
important for several issues, including the protection of riparian (streamside) resources,
protection of wildlife habitat (for example, the instream habitat for the threatened Neosho
madtom, Noturus placidus), and bank stabilization related to property loss, general
aesthetics, and recreation. Also, such information may be important to the site selection,
design, and construction of future overflow dams.
The focus of this study was the 180-mile reach of the Neosho River between John Redmond Dam
and the Kansas-Oklahoma State line (fig. 2). Throughout
this reach the Neosho River is characterized by a meandering, gravel-bed channel. The channel
slope averages about 1.2 feet per mile, and bank height varies from about 15 to 30 feet. The
channel bed frequently is situated on erosion-resistant bedrock. Alluvium in the Neosho River
Valley averages about 25 feet in thickness and is typified by silt and clay with a basal
layer of sand and gravel that averages about 3 feet in thickness. The bank materials consist
mostly of cohesive silt and clay that are relatively resistant to erosion compared to sand
banks. Also, the banks typically have partial-to-complete mature tree cover that may enhance
or reduce bank stability at specific locations.
John Redmond Dam, located about 3 miles upstream from the Burlington over-flow dam
(fig. 2), was completed by the U.S. Army Corps of
Engineers in 1964. Downstream changes in the streamflow regime attributed to the operation of
John Redmond Dam have included a decrease in the magnitude of peak flows and an increase in
the magnitude of low flows (Studley, 1996). Also, post-dam suspended-sediment concentrations
have been substantially reduced immediately downstream from the dam. Farther downstream,
these effects are moderated by several tributaries that contribute unregulated flow to the
Neosho River. A recent study by Juracek (1999) found that the operation of John Redmond Dam
has had little effect on the stability of the downstream Neosho River channel.
Overflow dams result in upstream and downstream changes in a river channel. Upstream from the
dams a backwater pool is created, the depth and extent of which depends on the height of the
individual dam and the slope of the channel (fig. 3). Within this
backwater reach of the river, the decrease in flow velocity may result in some deposition of
material on the channel bed. As the water flows over the dam, its velocity and erosive power
increase. The potential consequences of these effects include increased channel bed and bank
erosion immediately downstream from the dam as the river expends the extra energy. Most of
the erosion occurs during high flows when the river's erosive power is greatest. Bed erosion
may create a plunge pool at the base of the dam, with the formation of companion depositional
bars just downstream as the river loses its ability to transport the scoured bed load
(fig. 3). The depositional bars may divert the flow toward or away
from the banks. Bank erosion results in channel widening and also may contribute to the
formation of the depositional bars. The form and downstream extent of these effects vary from
site to site as affected by human-caused and natural conditions that may either promote or
inhibit the river's response to the overflow dams.
Aerial photographs were used to assess the geomorphic effects of overflow dams on the lower
Neosho River channel. Bankfull channel width was used as the primary indicator variable to
compare channel form upstream and downstream from the dams. Analysis of channel change was
completed using winter 199091 leaf-off color aerial photographs (scale 1:12,000) that
were enlarged from 35-millimeter slides (fig. 4). The 199091 photographs were taken
during a period of low flow, and thus channel features were more visible. For each site,
bankfull channel width was interpreted approximately 1 mile upstream and downstream from the
dam, traced on a scale-stable mylar overlay, and digitized. Primary indicators used in the
delineation of bankfull channel width included breaks in slope, the tops of point bars, and
changes in vegetation.
The aerial photographs, combined with onsite inspection, were used to determine the location
and type of human-caused or natural conditions that might obscure channel response in the
vicinity of the dams. Human-caused conditions included structures in or along the channel and
channel modifications such as the addition of riprap (large rocks used to protect banks from
erosion). Natural conditions included split channels, hard points (that is, locations where
the channel is situated along the valley wall), and bedrock.
Analysis of the photograph-derived channel information included an assessment of the changes
in bankfull channel width and the shape of the channel upstream and downstream from the dams.
These measurements then were used to estimate the upstream and downstream extent of variation
attributed to the dams. To the extent possible, older aerial photographs and onsite
inspection were used to assess the stability of the channel changes over time. The results
then were interpreted in relation to site-specific human-caused and natural conditions that
may affect the channel response. In the following discussion, the designation of right and
left channel bank was made looking in the downstream direction.
Results of the aerial-photograph analyses indicate that the geomorphic effects of the
overflow dams appear to be limited to a short reach of river channel located immediately
downstream from the dams. This geomorphic-response zone typically is confined to the first
1,000 feet downstream from the dams. Within the geomorphic-response zone at each site,
channel widening has occurred to some degree (fig. 5), except
where limited by riprap, concrete wall, and (or) bedrock.
The most pronounced widening was observed at the Burlington and Neosho Falls sites, with
approximate dam construction dates of 1900 and 1935, respectively. The widening at these two
locations may be attributed in part to the relatively old age of the overflow dams (that is,
time for geomorphic response) and the absence of extensive riprap, concrete wall, or bedrock
on the channel banks downstream from the dams. The bridge and associated riprap just
downstream from the dam at Neosho Falls were not constructed until the mid-1970's therefore,
about 40 years of geomorphic-response time had transpired at this location prior to the
construction of these features.
At Burlington, three additional factors may have contributed to the channel widening. First,
the height of the dam is about three times greater than the typical height of the dams at the
other 11 sites. Thus, the water flowing over the dam likely achieves greater velocity and
erosive power. Second, the width of the dam is substantially greater than the apparent
original channel width, and this may have contributed to the pronounced widening at this
location. Finally, the formation of a large tree-covered rock bar likely contributed to the
channel widening by diverting flow toward the banks.
Substantial channel widening in the geomorphic-response zone also was observed at the
Humboldt, Chanute Northeast, Erie, and South Mound sites (fig.
5). At Humboldt and Erie, both banks were stabilized with riprap at some undetermined
date. However, the shapes of the channel indicate that considerable bank erosion had occurred
before the riprap was added. At the South Mound site, the rapidly migrating channel eroded
around the dam on the right bank in 1995. It appears that, unless stabilized, the channel
eventually will completely bypass the dam at this location.
Moderate to minimal channel widening was observed in the geomorphic-response zone at the
remaining sites. At Iola, additional erosion of the right bank is limited by riprap as well
as a rock bar that may have been artificially constructed to divert flow away from the bank.
At the Chanute Southeast site, a small area of active cutbank erosion on the right bank
appears to be due in part to the diversion of flow around an in-channel concrete structure
just downstream from the dam. Otherwise, channel widening at this site is minimal. The 1991
aerial photograph showed only a slight widening of the channel in the response zone at
Oswego. However, recent onsite inspection (February 1999) discovered rapid left-bank erosion
in progress within the response zone (fig. 6). The erosion at this site had removed all tree cover along the affected
bank and was advancing into cropland.
At some locations, such as the Montana and Chetopa sites, channel change in the
geomorphic-response zone has been limited by some combination of riprap, concrete wall, and
(fig. 5). The widening evident on the left bank
downstream from the concrete wall at the Montana site appears to be the result of the
original construction design rather than a reaction to bank erosion. At Chetopa, the riprap
on the cutbank side (right bank) of the channel immediately downstream from the dam may have
diverted the bank erosion to the opposite bank, resulting in the widening evident on the left
bank farther downstream (fig. 5). Onsite inspection indicated
at least some bank erosion immediately downstream from riprap at all 12 sites.
The effects of the overflow dam at the Chanute North site appear to be indeterminate due to
riprap and the location of the dam with respect to the natural cutbank (right bank)
(fig. 5). Given that maximum erosion tends to occur on
the outer bank just downstream from the apex of a channel meander, the erosion observed on
the right bank downstream from that dam may be due simply to normal cutbank erosion. At this
and other sites, the control afforded by the dam and associated riprap may be inhibiting
The stability of the channel change in the geomorphic-response zone was difficult to assess
due to a lack of older leaf-off aerial photography and uncertainty regarding the timing of
riprap construction in relation to the date of dam construction. However, there is some
evidence to suggest that, where not inhibited by riprap, concrete wall, or bedrock, channel
changes at several dam sites are in progress. Typically at these sites, steep erodible banks
are being replaced by gentle, stable banks within the response zone
(fig. 7). The steep erodible banks appear to consist mostly of
silt, whereas the gentle, stable banks appear to consist mostly of coarser grained sand and
gravel. These channel changes were particularly evident at Neosho Falls. Eventually, where
site conditions permit, the channel within the response zone may evolve into an approximately
stable form characterized by a larger bankfull channel width and gentler bank slopes compared
to the channel immediately upstream and downstream from the response zone.
Aerial photographic evidence indicated that most of the overflow dams have had substantial
geomorphic effects on the Neosho River channel. Examples of such evidence include channel
widening and (or) the presence of channel gravel bars immediately downstream from the
majority of the dams.
Overall, the combination of aerial-photograph analyses and onsite inspection indicated
moderate to pronounced channel widening in the geomorphic-response zone immediately
downstream from the overflow dam at 8 of 12 sites. Also, within the response zone, steep
erodible banks are gradually being replaced by gentle, stable banks. Where uninhibited by
riprap, concrete wall, and (or) bedrock, channel adjustment within the response zone is
ongoing. Conversely, where riprap, concrete wall, and (or) bedrock are in place, channel
adjustment has been effectively minimized. However, as evidenced at Chetopa, it is possible
that the suppressed channel change may reappear farther downstream.
This study describes the potential geomorphic consequences of constructing an overflow dam
across a river channel. Results indicated that localized human-caused and natural conditions
can substantially affect the river channel's response to an overflow dam. The information
provided is important for several issues, including the protection of riparian resources and
habitat, bank stabilization, and the site selection and design of future overflow dams.
The study described by this report was done by the U.S. Geological Survey in cooperation with
the Kansas Water Office and was supported in part by the Kansas State Water Plan Fund. The
author also gratefully acknowledges the aerial photographs provided by the U.S. Department of
Agricultures Farm Service Agency.
- Juracek, K.E., 1999, Channel stability of the Neosho River downstream from John
Redmond Dam, Kansas: U.S. Geological Survey Fact Sheet FS-088-99, 4 p.
- 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 p.
For more information please contact:
U.S. Geological Survey
4821 Quail Crest Place
Lawrence, Kansas 66049-3839