EXPLORATORY STATISTICAL AND SPATIAL MODELS OF UNIONID 
MUSSEL DISTRIBUTIONS IN A REACH OF THE UPPER MISSISSIPPI RIVER

Steven J. Zigler1, Teresa J. Newton1, Jeff J. Steuer2, Michelle R. Bartsch1, and Jennie S. 
Sauer1 
1U.S.Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 
54603   2U.S. Geological Survey, Wisconsin Water Science Center, Middleton, WI 53562

We assessed whether the spatial distribution of unionid mussels could be predicted from 
physical and hydraulic variables in Navigation Pool 8, Upper Mississippi River.   
Exploratory classification and regression tree (CART) models were constructed using 
mussel data compiled from various sources, and explanatory variables derived from GIS 
coverages.  Prediction success of CART models for presence-absence of mussels ranged 
from 71 to 76% across three gears (brail, sled-dredge, and dive-quadrat).  Models were 
largely driven by shear stress and substrate stability variables, but interactions with 
simple physical variables, especially slope, were also important.  Geospatial models, 
which were based on tree model results, predicted few mussels in poorly connected 
backwater areas (e.g., floodplain lakes) and the navigation channel, whereas main 
channel border areas with high geomorphic complexity (e.g., river bends, islands, side 
channel entrances) and small side channels were typically favorable to mussels.   
Moreover, discharge-specific CART models suggested that episodic events such as 
droughts and floods were important in structuring mussel distributions in Pool 8.

Keywords: unionid, mussel, shear stress, tree model, geospatial