MEASURES OF RIVER-FLOODPLAIN CONNECTIVITY I: ISOLATED BACKWATERS IN SELECTED 
REACHES OF THE UPPER MISSISSIPPI RIVER.

Kenneth S. Lubinski1 and Jeffrey A. Yanke1,2.   1U.S. Geological Survey, Upper 
Midwest Environmental Sciences Center, La Crosse, WI 54602  2University of 
Wisconsin-La Crosse, La Crosse, WI 54601.

River-floodplain connectivity has been reported as influencing material 
transport and loading, animal movements, plant colonization, biodiversity, and 
production within many large river ecosystems, and consequently was recently 
described as one of six important criteria for evaluating Upper Mississippi 
River ecosystem health.  Surprisingly little attention, however, has been 
devoted to developing quantitative methods for measuring river-floodplain 
connectivity, or correlating resultant values to physical, hydrologic or 
biological conditions. To initiate a long-term science strategy on the 
ecological relevance of river-floodplain connectivity, and in the absence of 
detailed floodplain elevation information, we focused first on describing 
present and past spatial characteristics of isolated backwaters.  The number, 
area, and location of isolated backwaters, by size class, in five river reaches 
were quantified from maps representing conditions that existed in 1989 and 
1890's.   In 1989, isolated backwaters comprised between 1.9 and 3.8% of total 
mapped water in the reaches. In all reaches, the largest numbers of isolated 
backwater fell within the smallest (less than 1 ha) size classes.  Cumulative 
area curves, however, indicated that isolated backwaters less than 2.9 ha in 
size made up less that 40, 31, 28, 27, and 18 percent of the total area of 
isolated water in Navigation Pools 4, 8, 13, 26, and the Open River, 
respectively.  In Navigation Pools 4, 8, and 13, size frequency curves of 
isolated backwaters in 1890 were similar in pattern, but consistently lower, 
than those in 1989.  In Navigation Pool 26 and the Open River, isolated 
backwaters in 1890 were too rare to support a general characterization of number 
or total area by size class.  The lack of isolated backwaters in these reaches 
was likely related to the early conversion of floodplain land to agriculture.  
Patterns of size frequency and cumulative area by size class among the reaches 
suggest that these spatial variables may be consistent among other river reaches 
as well.  However, the inverse relationship backwater number and size may 
present a potential dilemma for pool-scale water level management strategies 
that seek to affect a maximum number of isolated backwaters, versus those that 
seek to affect the greatest total area.


Keywords:  connectivity, Mississippi River, isolated backwaters, floodplain, 
ecosytem health