SYTHESIS OF UPPER MISSISSIPPI RIVER SYSTEM VEGETATION:  PAST, PRESENT, AND 
FUTURE

Susan P. Romano1, Yao Yin2, Megan Moore3, and Thad Cook4

Historical accounts of the Upper Mississippi River (UMR) indicate a diverse 
environment of inundated floodplains, numerous channels, and temporary ponds.  
These supported high species richness of both submersed (23+spp) and emergent 
(22 – 80+spp) aquatic vegetation.  Following construction of the locks and 
dams in the 1930’s, the floodplains became permanently flooded, allowing 
submersed and emergent aquatic vegetation to flourish in these vast new 
shallow aquatic areas.  Impoundments resulted in the loss of the natural 
hydrograph, and increased sedimentation, wave action, and nutrient 
enrichment.  Aquatic vegetation began to disappear on the UMR under the 
impacts of these stressors.  Recent studies of aquatic vegetation conducted 
on the UMR have shown that species richness of submersed macrophytes (21spp) 
and emergent vegetation (14 – 80+spp) has decreased very little from pre lock 
and dam conditions.  However, abundance of aquatic macrophytes has been 
drastically reduced and species composition has been highly altered.  
	The Illinois River was also historically abundant in aquatic 
vegetation, particularly submersed aquatic vegetation (SAV), in the lower 
Illinois River and connected floodplain backwaters during the early 1900’s.  
However, as early as 1915, lush beds of SAV began to disappear from the 
backwater lakes and the main-stem river following the diversion of Lake 
Michigan waters to the Illinois River.  By the 1950’s, SAV had almost 
completely disappeared from these habitats within the Illinois River.  Recent 
studies indicate an increase in SAV in the upper Illinois River, in the 
Dresden Pool, south of Joliet, Illinois.  Currently, virtually no submersed, 
rooted floating-leaved, or non-rooted floating-leaved vegetation is found 
further downstream within the Marseilles and Starved Rock Pools of the upper 
Illinois River, and within main stem river habitats of lower Illinois River 
pools, including La Grange Pool.  Lack of SAV within these lower Illinois 
River pools may be explained through longitudinal variability in river 
condition among pools.  Water clarity is greater and water levels are more 
stable in the upper Illinois River pools compared to the lower pool.  Future 
research should focus on determining limiting and promoting factors for SAV 
establishment and growth along a longitudinal gradient of pools in the 
Illinois River.  
The silver maple – American elm forest extends throughout the floodplains of 
the UMR System.  Historically, some forest removal occurred because 
steamboats were fueled by wood, primarily from the accessible forests that 
lined the river system.  Agricultural expansion followed the harvest and 
clearing of the floodplain forest.  Over 50% of the river floodplain was 
converted to agriculture in some areas as agricultural levees were built for 
water control.  Currently, most floodplain forests are less diverse, 
dominated by silver maple. Some recent changes in floodplain forest species 
composition between the mouth of the Missouri River and the mouth of the Ohio 
River were caused by the Great Midwest Flood of 1993.  During the flood, 
floodplain forests were inundated at unusually high water levels and for a 
rare and lengthy duration throughout the growing season.  The flood reduced 
tree density, killing particularly small and weak trees, and less water 
tolerant species such as hackberry and pin oak.  This provided an opportunity 
for eastern cottonwood and black willow to regenerate in this portion of the 
Mississippi River.  Floodplain forests above the mouth of the Missouri River 
continue to be dominated by silver maple.  Future floodplain forest research 
should include studies of species composition changes related to flooding, 
management, river and land use, groundwater loss, and changes in temperature 
and precipitation related to global warming.