INCREASED FLOODING PORTENDS INCREASED  IDSTURBANCE  FOR  FLOODPLAIN
FORESTS

Joseph H. Wlosinski and Yao Yin.
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta 
Reed Road, La Crosse, WI  54603.

Investigations after the 1993 flood on the Upper Mississippi River revealed a strong 
correlation between flood duration and mortality of trees on the floodplain.  In areas 
near St. Louis, Missouri, mortality of mature trees, measured in 1994, was 37%, 
mortality of 2-10 cm diameter saplings was 80%, and no seedlings survived.  We 
investigated changes on flood characteristics and here discuss the potential effects of 
such changes on floodplain forests.  This analysis is limited to areas between the 
levees.

We studied three water surface elevation gages from the Upper Mississippi River 
System (UMRS) that have a nearly complete daily record for 99 years or more; 
Muscatine, Iowa (1901-1998), St. Louis, Missouri (1861-1998), and Valley City on 
the Illinois River (1885-1998).  We examined trends in flood heights, flood duration, 
and flood frequency.  Although the data were extremely variable, significant trends in 
all three factors were evident at all three stations.  The average flood height, from 
linear regression analysis, is now about 1.5 meters higher than when these data were 
first collected.  Flood frequencies are also greater.  After dividing the record for each 
station into two equal periods, we calculated the number of floods that occurred in 
each period.  Twenty floods occurred at St. Louis during the early 69 year period and 
33 during the later years.  Numbers of floods increased from 14 to 28 at Muscatine 
(57 year periods) and from 18 to 37 at Valley City (49 year periods).  The 10 greatest 
floods, as measured by water surface elevations, occurred in the later period at all 
three stations.  Flood duration was also longer in the latter periods.  Linear regression 
showed that the number of days above flood stage in a year increased between 18 
and 35 days over the period of record.

We believe that these changes in the hydrograph will lessen forest regeneration after 
major floods, because seedlings and saplings fully submersed for a week usually 
suffer 100% mortality.  With floods now occurring twice as often and having a longer 
duration, seedling success is lower.  Higher floods will also negatively affect a larger 
area of the floodplain.  We predict that flood-tolerant and faster growing species will 
start to dominate higher and higher elevations, and modern forest canopies will be 
more open than their historical counterparts.  Forest restoration can be improved by 
incorporating these results in future planning efforts. 

Keywords:  Upper Mississippi River, hydrology, floods, flood-plain forest
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