A GIS TECHNIQUE FOR ASSESSING SEDIMENT HARDNESS IN A MANAGED ILLINOIS RIVER 
BACKWATER

James T. Lamer, Chad R. Dolan, and John H. Chick
Great Rivers Field Station, Illinois Natural History Survey, Brighton, IL  62012

Swan Lake is located at Illinois River Mile 5-12 in Calhoun County, IL, and is 
managed by USFWS/IDNR staff.  This floodplain lake was created in 1938 after 
construction of L&D 26 on the Mississippi River.  Sedimentation and lack of 
summer dry-periods led to flocculent substrate, high turbidity, and overall 
degradation of the system.  The USACE implemented a habitat rehabilitation and 
enhancement project (HREP) in 1993.  Goals included consolidating sediment to 
promote the growth of aquatic vegetation, and subsequently, benefit fish and 
wildlife.  Swan Lake was divided into a lower, middle and upper unit, and stop-
log gates were installed to allow for water-level manipulation.  The upper unit 
has been subject to annual draw-downs since before HREP construction; draw-downs 
in the lower and middle units have been less regular.  Consolidated sediments 
may provide suitable conditions for colonization of submersed and emergent 
vegetation.  Agencies involved in HREP activities are interested in the success 
of draw-downs or regression of consolidated areas that remain inundated.  Using 
GIS, sediment hardness maps were constructed to develop a baseline to which 
future data can be compared to quantitatively assess draw-down success within 
Swan Lake.  Sediment hardness across the three units was found to be 
significantly different (P<0.01; n=607).  The upper unit had the lowest sediment 
penetration (mean=10.9 cm), the lower unit the highest (mean=43.9 cm), while the 
middle unit is intermediate (mean=24.6 cm).  Adaptive management is being 
implemented within Swan Lake, so evaluating the success of draw-downs may be 
critical in determining future management strategies.

Keywords:  sedimentation, Swan Lake, hardness, GIS, adaptive management