WATER LEVEL MANAGEMENT AND HYDROLOGIC DISTURBANCE GRADIENTS IN BACKWATERS OF A 
MISSISSIPPI RIVER NAVIGATION POOL: RESPONSES OF MACROINVERTEBRATES AND BENTHIC 
ORGANIC MATTER 

Michael B. Flinn1, Matt R. Whiles1, S. Reid Adams1,2, J. E. Garvey1,2
1Department of Zoology, Southern Illinois University, Carbondale, IL 62901
2Fisheries and Illinois Aquaculture Center, Southern Illinois University, 
Carbondale, IL 62901
 
The St. Louis District of the Army Corps of Engineers implemented Environmental 
Pool Management in 1994 on Mississippi River Pools 24, 25, and 26.  The 
operational goal of this water level management is to maintain relatively low, 
stable water levels in the lower portion of the pools, following maximum 
drawdown in the spring, in order to better simulate a natural hydrograph without 
inhibiting commercial navigation.  When possible, water levels are held up to 
2.0 feet below the target pool elevation at the lock and dam for at least 30 
days.  Previous investigations of mudflats exposed in the lower pool via water 
level management showed substantial production of emergent vegetation consisting 
primarily of millet, chufa, and smartweeds; however, there is little information 
regarding the responses of consumer groups. 

The influence of water level management on backwater habitats in Mississippi 
River Pool 25 varies along the length of the pool and creates a hydrologic 
disturbance gradient whereby mid-pool backwaters dry for short periods (~10 
days/yr) and lower-pool backwaters dry extensively (~60 days/yr), mainly during 
summer.  We examined the influence of this gradient on organic matter and 
macroinvertebrates during 2001-2003. Three samples were collected seasonally at 
three mid-pool and four lower-pool sites using a stovepipe sampler. Benthic 
organic matter values were generally stable at mid-pool throughout the year, but 
lower-pool values were variable because of significant fluctuations in CPOM 
(P<0.01), especially in fall after vegetation grew on exposed mudflats. Lower-
pool habitats had generally higher macroinvertebrate taxa richness, with 
significantly higher values in spring 2002 and fall 2003 (P<0.05), and responses 
of individual taxa varied.  Lower-pool habitats were dominated by multivoltine 
taxa in spring, but multivoltine groups had higher relative abundance and 
biomass at mid-pool in fall following summer drying. For example, Oligochaeta 
and Chironomidae abundance and biomass were higher at lower-pool in spring 
(P<0.05), but both had higher abundance and biomass at mid-pool in fall 
(P<0.05). Univoltine taxa preferred longer hydroperiods typical of mid-pool.  
For example, Hexagenia abundance and biomass were higher in mid-pool habitats 
throughout the study (P<0.01). Results show that water level management in this 
navigation pool influences backwater organic matter and macroinvertebrates and 
may increase diversity at the pool scale because of the gradient of hydrologic 
conditions it creates in backwater habitats.

Keywords:  macroinvertebrates, disturbance gradients, organic matter, 
hydroperiod, water level management