INTO THE NEW MILLENNIUM:  CONTINUED EFFECTS OF THE 1993 FLOOD ON
FLOODPLAIN FORESTS, LA GRANGE REACH, ILLINOIS RIVER.

Thad R. Cook and Mark A. Pegg.  
Illinois Natural History Survey, Illinois River Biological Station, Long Term Resource
Monitoring Program, Havana, IL 62644.

In 1993, the Upper Mississippi River System experienced an extreme flood.  Staff at the Illinois
River Biological Station as part of the Long Term Resource Monitoring Program began a study
of the effects of this significant disturbance to the floodplain forests of the Illinois River's La
Grange Reach (RM 80-158), to assess post-flood tree mortality and monitor forest community
changes.  In 1994, 14 transects were established perpendicular to La Grange Reach of the Illinois
River and across the floodplain to monitor and document these changes.  In 1995, monitoring
continued at 46 random sites within the La Grange Reach.  In the spring of 2000, the same 46
sites were re-visited following the same criteria used in 1995.  In addition, we took cores and
measured canopy height of two over story trees at each site in 2000.  The 1995 and 2000 sites
will be revisited during flood stage to gather elevation data at a later date.  This data may be used
to correlate age and growth with past hydrology.  Data from 1994 and 1995 were combined and
indicated tree mortality for all species combined was 28.2%.  Simpson's Diversity Index showed
only a slight decrease in species diversity as compared to pre-flood conditions.  Trees in smaller
diameter classes had a higher mortality than larger diameter classes.  Importance Values (IV)
indicated Silver Maple (Acer saccharinum) to be the most important mature tree species and to
have increased in dominance following the 1993 flood.  Willow species (Salix spp), the second
most abundant species, decreased in dominance in 1995.  Data for 2000 should reflect the same
trends as 94 and 95.  Late season flooding in years following 1993 has put additional stress on
trees already weakened causing increased mortality to floodplain trees. It appears that
regeneration of the floodplain forests is doing well, but the quality of the tree community that is
replacing the older forests is in question.

Keywords: floodplain forest, Illinois River, mortality, flood, hydrology