FLOOD04610

• County: Statewide
• Basin: Statewide
• Title: Overview of River-Floodplain Ecology in the Upper Mississippi River Basin
• Date: 12/9/1996
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

18

OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN

_ 162 -Rivermile366.0(588.9km)
~
~ 160
~ Iowa
3l 158 SW
c
~
~ t
E 156

i 154 \.
~ 152 ,
1'50 ~'

.~ 148 1-
o
~ 146
W 144 -

Illinois
NE,
1983~

1946

.1936
.1928"

1891

__1_ -" I I
o ~ ~ ~ ~ 1~1,~ 1~1~

Distance from right bank of river, in meters

Figure 2-9. Riverbed elevations through time at Mississippi
River mile 366 (Pool 19). Because UMR navigation dams slow
river current velocity, sediments drop out of suspension and build
up on the river bottom. This effect is most severe in slow-flowing
aquatic habitats created by navigation dams. Although sedimenta-
tion is a natural process, the effects have been intensified by high
soil erosion and hydrologic perturbations throughout the basin
(source: Bhowmik and Adams, 1989; reproduced with permission
of the author).

species lists are presented by Fremling and others (1989)
and Rasmussen (1979), They were recently updated for the
entire UMRS (Pitlo and others. 1995).
In summary. the upper floodplain reach has the least
disturbed floodplain habitat because of the presence of the
Upper Mississippi Fish and Wildlife Refuge, Aquatic habi-
tats. however. have been degraded by the negative effects of
impoundment. sedimentation, and water-level regulation.

LOWER FLOODPLAIN REACH (POOLS 14-26)

RIVER ENGINEERING

Twelve navigation dams were constructed on the lower
floodplain reach between Clinton. Iowa. and Alton. Illinois,
Lock and Dam 26 also maintains navigation along the 80-
mile reach of the Illinois River below LaGrange. Illinois,
Water levels in Pools 16 and 24-26 are controlled at mid-
pool control points. Drawdowns are a maximum of 2.7,4.3,
and 6,7 feet in Pools 24. 25. and 26, respectively, Other
pools controlled in this manner have drawdowns of less
than I foot. Approximately 40 percent of floodplain habitats
are in public ownership (UMRBC. 1981), Levee districts
have sequestered about 43.000 acres of floodplain habitat in
this reach,
The effects of navigation dams were similar to those in
the upper floodplain reach (see fig. 3...{i in Chapter 3). Pre-
dam (1891) and postdam (1989) land-cover/land-use maps
of Pool 26 reach show agricultural development in 28 per-
cent of the floodplain prior to 1900, In the following 90

"

years. a 5 percent increase in tillable acreage probably elim-
inated the last mesic prairies and marshes. When the dams
began operation in 1938. most sand and mud habitats and
many islands were permanently inundated. Significant new
permanent aquatic habitats were created where diverse
floodplain wetlands had been; Swan Lake alone accounts
for much of the increase. Abundant plant production on ter-
restrial portions of the floodplain probably supported high
aquatic production when inundated in the past. Swan Lake
and most other off-channel aquatic habitats in the lower
floodplain reach currently suffer from high rates of sedi-
mentation due to erosion from a predominantly agricultural
basin.
In general, there is more open-water aquatic habitat in
the lower and middle reaches of the pools. and islands are
less numerous than in the late 1800's (Simons and others.
1975). Sedimentation from intense agricultural develop-
ment within the basin fills shallow backwaters. produces
poor sediment quality. and leads to high rates of sediment
resuspension from wind- and boat-generated waves. In
pools operated at mid pool control points, drawdowns con-
solidate sediments in lower pool reaches.

WETLAND HABITATS

As in upstream reaches. aquatic plants thrived in the
newly created backwater lakes (R, DeShirlia. personal com-
mun" 1993). The plants have been severely reduced by
excessive sedimentation and occur primarily in isolated
backwaters managed for waterfowl (fig, 3...{i in Chapter 3),
In periods of drought. when sediment transport is lower and
water levels are most stable, aquatic plants are more abun-
dant and more widely distributed (Theiling and others.
1996). Emergent plants are most common in managed back-
water habitats and in drawdown-affected areas.

INVERTEBRATES

Benthic invertebrates occur in lower densities in the
lower floodplain reach than in the upper floodplain reach
(LTRMP, unpub. data). No comparisons of epiphytic or
epilithic invertebrates are available. but I suspect that densi-
ties of these groups are higher in the north as well. Poor
water and sediment quality and a lack of plants contribute to
the lower abundance of aquatic invertebrates in the south. [n
Swan Lake (Illinois River). however. high densities of large
midges are common and are probably an important food
source for fishes.

FISHES

Fish populations expanded considerably after
impoundment. Sportfishing was so good that tourism helped
the economy of Grafton, Illinois (R. DeShirlia. personal
commun,. 1992). Commercial fishing also contributed a