SUSTAINED HIGH WATER ON THE MISSISSIPPI RIVER: INFUENCES ON FISH ASSEMBLAGES IN 
COLDWATER TRIBUTARIES.

Neal D. Mundahl.
Department of Biology, Winona State University, Winona, MN 55987.

The Mississippi River at Winona, MN, remained at flood stage for one month (12 
April-13 May) during 2001, but high water levels were sustained for three months 
(7 April-5 July).  High waters flooded low islands and inundated the lower 
reaches of many coldwater tributaries.  This study examined the influence of 
this inundation on the fish assemblages normally found in these coldwater 
streams.  Five coldwater tributaries with survey data from previous years were 
electrofished after water levels returned to normal in late-summer 2001, and 
four of these sites were sampled again in 2002.  Twelve species of river fish 
(one to four per stream) not normally found in these streams remained after 
waters receded in 2001.  These “new” fish comprised 10.6-51.6% of the total fish 
collected from each stream.  Northern pike (especially YOY fish) was the most 
common invader, present in four of the streams.  Other new species included 
golden redhorse, quillback, sauger, crystal darter, spotted sucker, and 
goldfish.  The presence of these fish in the coldwater streams in 2001 slightly 
reduced the biotic integrity scores of these streams relative to pre-inundation 
surveys.  Fish assemblages had returned to normal during 2002 surveys, with only 
a few token individual river fish remaining in two of the streams.  Although 
inundation by Mississippi River floodwaters may temporarily alter the fish 
assemblages in coldwater tributaries, the effects appear to be minimal and the 
recovery rapid.

Key words: flooding, tributary streams, fish assemblages, coldwater streams, 
biotic integrity