HYDROLOGIC AND HYDRAULIC DRIVERS OF THE UPPER MISSISSIPPI RIVER ECOSYSTEM

Charles Theiling

U.S. Army Corps of Engineers, Rock Island District, PO Box 2004, Clock Tower 
Bldg., Rock Island, IL 61204

River hydrology, in the broadest sense, is perhaps the strongest driver of 
floodplain river ecosystems.  Hydrologic and hydraulic (H&H) processes drive 
sediment transport which shapes the channels and backwaters of aquatic 
habitats and forms the terrestrial geomorphic template that supports diverse 
plant and animal communities.  River flow also transports dissolved nutrients 
and organic matter, including large woody debris, coarse and fine 
particulates, gametes, eggs, larvae, and invertebrates.  H&H processes are 
important factors determining plant and animal habitats, especially soil 
moisture gradients ranging from permanent aquatic to rarely flooded and that 
support a range of aquatic to terrestrial upland plant species in relatively 
distinct community types along inundation gradients.  Rheophylic fishes and 
macroinvertebrates exploit swift channels, while lentic fishes use off-
channel aquatic habitat, and diving/swimming birds eat plants, invertebrates, 
or fishes in deepwater while dabblers and waders eat plants, seeds, 
invertebrates, amphibians, and fishes in shallow water and mudflats.  Low 
flow H&H zonation changes with flooding, and mobile critters migrate with 
floodwaters to exploit seasonal habitats.

UMR hydrology is well understood with regard to navigation and flood control, 
but less well understood from an ecological perspective.  River hydraulics 
have been used since the mid-19th Century to shape a deep and stable main 
navigation channel by clearing and constricting the channel.  Large scale 
flood control and floodplain drainage and development started in the late 
19th Century.  Low flow river stage regulation to maintain a minimum 9-ft 
navigation channel was achieved by a network of 37 low-head navigation dams.  
Upland development has change the rate and volume of runoff that reaches the 
mainstem.  Each of these developments has caused significant ecological 
change, sometimes with very different effects depending on location along the 
1,200 miles of river or the lateral channel-floodplain gradient.  

River ecologists and conservationists have recommended naturalizing the 
hydrologic regime with regard to five ecologically relevant characteristics: 
magnitude, frequency, duration, timing, and rate of change.  These parameters 
are described systematically for the UMR because there is wide availability 
of river discharge and stage data.  Long term discharge and stage data 
spanning pre and post development are available for nine gauges throughout 
the system.  Each navigation reach also has three or more gauges to assess 
post dam, riverine to lentic hydrologic gradients in impounded reaches.  
River stage-discharge modeling has been used to estimate floodplain 
inundation for flood protection for decades, and the ecological significance 
of flooding has been generally described, but UMR flood inundation mapping 
has not been evaluated for ecological significance.  Recent flood inundation 
analysis on high resolution topographic data has greatly increased 
opportunities to expand pilot study analyses.  Historic assessments of flood 
inundation to assess change due to development has also not been completed, 
but appropriate map and hydrologic data are widely available and 
demonstrations have been extremely informative.  The five ecologically 
relevant hydrologic characteristics can also be applied to area of inundation 
when it is clearly mapped over floodplain elevation data.  River hydraulic 
characteristics operate at fine scales and can vary widely with river 
discharge.  Advanced 2-dimensional hydraulic models developed for many river 
reaches have been used to design navigation and ecosystem restoration 
features, and only recently to describe freshwater mussel habitat.  These 
models can be used more widely to estimate habitat availability, material 
transport capacity, and other ecosystem processes in other river reaches.

Keywords:  hydrology, hydraulics, inundation mapping, ecohydrology, 
floodplain river