Located in the lower reaches of the Otter Creek watershed, Otter Creek Marsh comprises some 3,360 acres near Chelsea in southeast Tama County.  The casual observer visiting Otter Creek Marsh during the summer is usually amazed at the lack of water in the marsh.  The reason for this low water condition is that marsh managers try to duplicate natural processes of wet-dry cycles.  Mother Nature frequently takes 2-5 years to complete the cycle, and we accomplish it (usually) in about six months.  Mother Nature’s wet and dry cycles can cause drastic changes in wetland habitat.  Water fluctuations frequently create undesirable marsh conditions for an extended period of time.  To counter this situation, dikes and water control structures can be constructed to provide a more positive form of water level control.  These dikes and control structures allow the marsh manager to adjust his form of management to the conditions at hand, instead of waiting for nature to take its course. 

A manager has certain objectives in mind when manipulating water levels.  Many of these objectives are the same, whether managing a 25-acre marsh along a creek or a 3,360 acre wetland complex on the Iowa River floodplain. 

Some of these objectives are:
1.  Increase waterfowl breeding habitat by increasing water depths to create a better interspersion of cover.
2.  Kill out sedge-grass monotypes or willows by flooding to bring about a succession of plants from moist soil and upland types to shallow and deep-water aquatics.
3.  Increase the production of natural food (such as smartweed) for waterfowl by the use of growing season drawdowns.
4.  Draw the marsh down in the summer to provide mudflats for artificial seeding of moist soil food plants, such as millet or wheat.
5.  Use early spring drawdowns to make it possible to till the bottom for planting         waterfowl food such as corn.
6.  Re-vegetate areas with emergent cover that were lost due to wave and ice         action, muskrat activity, or continuous deep water flooding.
7. Regulate muskrat populations and activity according to the management goals of the area.
8. Manipulate water levels to attract shorebirds to mud-flat conditions.

Water level management techniques vary from area to area depending upon the primary use of the wetland and its potential.  Each area has its own set of variables, including precipitation, runoff, watershed ratio, pumps, control structure capabilities (for getting water out and retaining water), water quality, bottom soils, rough fish populations, etc.

Some points to consider concerning water level manipulation are:
1. What is the management goal of the area?  Is it for waterfowl production and/or harvest?  Is it specifically for ducks, geese, muskrats, shorebirds, or fish and other animal species?
2.  What is the limiting factor affecting wildlife numbers?
3.  What is the anticipated goal or result desired from a drawdown?
4.  How will water levels be regained and at what time?
5.  Is there a fishery present and is it detrimental or compatible?

There is not always a simple answer to these questions.  Many times more than one question has to be answered, and the result may or may not be desirable.  The individual manager has to make the right decision based on local problems and past history, getting as much input as possible from interested parties in his local area.

The purpose of water level manipulation on any area is to cause change to occur.  Water levels can be raised or lowered to bring about the desired effect on vegetation, muskrats and fish, all of which are closely inter-related in management work.

The 50-50 cover-water ratio (hemi-marsh stage) is probably the most desirable condition to achieve.  It provides good conditions for hunter use with scattered openings to attract and disperse hunters and waterfowl.  But the marsh manager realizes that he is dealing with a dynamic habitat.  For this reason, it is not possible to maintain a specific condition of water and vegetation over a long period of time.  The overall purpose of water level manipulation is to prolong desired stages and shorten or eliminate certain less desirable stages.

Today’s marsh manager faces another challenge due to intensive drainage of adjacent agricultural lands.  Runoff from precipitation previously filtered through watershed systems and into marshes all summer long, resulting in a more even water input through dry periods.  But modern tiling and ditching systems now cause runoff to leave the agricultural land at a rapid pace, and in some watersheds late summer runoff has about been eliminated.  This has created some problems in refilling marshes after drawdowns.

On some areas, such as Otter Creek Marsh, pumps are used to supplement the lack of precipitation.  These pumps are expensive to run and maintain, but are sometimes the only feasible alternative.  Many of our major waterfowl areas in the State now use pumping systems of one kind or another.

A typical summer drawdown at Otter Creek Marsh is as follows: With the aid of water control structures, the marsh water levels are lowered in the spring.  This process creates broad areas of mudflats which are appealing to many species of wildlife (e.g. shorebirds, etc.).  Before long these mudflats turn into a sea of green as moist soil plants germinate and begin to grow.  By mid-June, lush growths of smartweed, arrowhead, cattail and other emergent vegetation cover the marsh.  The next step is getting water back into the marsh to flood the new growth so that ducks have access to it.  This is accomplished by closing the water control structures and diverting water from Otter Creek into the marsh segments.  Hopefully, late-summer rains will be adequate to replenish the water supply, thereby allowing us to fill the marsh segments.  If there isn’t adequate rainfall, pumping from the Iowa River is necessary.  The cycle from dry marsh to an average 2 1/2 foot deep marsh is completed in about six months.

So remember, the next time you drive by your favorite marsh and see nothing but mudflats, there’s probably a good reason for it.  The habitat in that marsh will most likely improve in the months ahead, and chances are good that duck hunting on the area will improve also. 

For further information regarding summer drawdowns on state wildlife management areas contact the Wildlife Biologist in the Management Unit that interests you.

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