ANALYSIS OF GAGE RECORDS TO  IDENTIFY  AND  QUANTIFY  CHANGES  WITH
"INDICATORS OF HYDROLOGIC ALTERATION" (IHA) SOFTWARE

Chuck Theiling 1, Scot Johnson 2, and Joseph H. Wlosinski 1.
1 U. S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta 
Reed Road, La Crosse, WI  54603; and 2 Minnesota Department of Natural Resources, 
1801 South Oak Street, Lake City, MN 55041.

River hydrology, as measured by changing discharges and water levels (stage), is the 
rhythm to which floodplain flora and fauna have found opportunities to evolve, 
survive, and proliferate.  As one of the primary drivers of the riverine ecosystem, 
changes in the hydrologic regime can alter aquatic habitats and their biotic 
inhabitants.  Fortunately, two of the longest data sets pertaining to the Upper 
Mississippi River System are river discharge and stage.  Until recently, use of these 
data sets by river scientists to evaluate changes in river hydrology were limited by 
available analytical methods and knowledge of ecologically relevant metrics.  Recent 
developments in scientific understanding of large river ecosystems has lead to the 
development of the Indicators of Hydrologic Alteration (IHA) software by The Nature 
Conservancy's Biohydrology Program.  Here we present initial results from our 
application of the IHA software to the Upper Mississippi River System discharge and 
stage data sets.

The IHA analysis calculates 33 statistical measurements for 5 ecologically relevant 
parameters: magnitude, duration, timing, frequency, and rate of change.  The IHA 
software uses linear regression techniques in trend analysis and uses parametric or 
percentile statistics to calculate a pre- versus post-impact comparison called the IHA 
Analysis.  In addition, the IHA software can be used to apply what is termed the 
"Range of Variability Approach" (RVA) to assess whether the post-impact statistics 
are outside the range of natural pre-impact conditions.  The RVA can be used to 
determine if alternative water regulation is needed to move the system toward a more 
naturalized flow regime.

Discharge data are available from the U.S. Geological Survey at 21 (14 active) gage 
locations, and stage data are available from the U.S. Army Corps of Engineers at 160 
(135 active) gage locations.  Data collection for most stations were started in the 
1930's, but daily data are available in electronic form for a few stations as far back 
as the late 1800's.  Records for gages distributed throughout the Upper Mississippi 
River were converted into the IHA format with SAS.  Changes at most stations 
included a reduction in the number and duration of low pulses, an increase in the 
number of high pulses, an increase in the number of reversals, and an increase in the 
rate of fall.  Critical examination of the IHA software output is required to interpret 
the results for any given gage in light of differences along the length of the system, 
tributary inputs, and intra and inter pool water regulations.

Keywords:  Upper Mississippi River, hydrology, statistical analysis, Indicator of 
Hydrologic Alteration, IHA, gage records
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