It’s not just my imagination.
The window of fishing opportunity in Iowa rivers and streams is steadily shrinking because the volume of water they carry is steadily rising.
I know for a fact I am fishing much less than I once did and much less than I would like. As a wading angler, whose efforts can be stymied by high, swift, muddy water, I have long believed my declining recreation has been caused by increasingly high flows in the Wapsipinicon, my favorite fishing hole, and in other Iowa streams.
That, too, is a fact, according to scientists at IIHR, Hydroscience and Engineering at the University of Iowa.
The Wapsipinicon discharges about twice as much water as it did in 1940, and the same is true for most other Iowa rivers, said IIHR research engineer Chris Jones.
Since 1935, the U.S. Geological Survey has been recording the Wapsipinicon’s discharge at DeWitt, near the river’s junction with the Mississippi.
From 1935 through 1954, the discharge averaged 1,401 cubic feet per second. During the most recent 20 years, from 1988 through 2017, that average had increased to 2,355 CFS.
As another illustration of the increased flow, Jones noted in 14 of the 55 years from 1935-1989, the annual average discharge was below 1,000 CFS, while in the 28 years since, that average has never fallen so low.
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As still another illustration, of the eight years with the highest average annual discharge, five have occurred in the past 10 years (2008, 2010, 2013, 2016 and 2017).
Jones said the five-year running annual average of discharge has increased about 2.5 times since 1940 — an increase similar to those of many other Iowa streams.
Jones said about 70 percent of the increased river flows can be attributed to more precipitation, which has increased about 8.4 percent during the last century statewide and in the upper Wapsipinicon watershed.
An increase in precipitation results in a much larger increase in stream flow, Jones said, primarily because so much of the excess finds its way into streams, either through runoff or groundwater contribution.
The post-1940 conversion of perennial vegetation to row crops, especially soybeans, accounts for most of the remaining increase in stream flow, he said.
Perennial vegetation, which is on the landscape the year around, releases much more water to the atmosphere via the process of evapotranspiration than do the shorter-lived soybeans, said IIHR hydrologist Keith Schilling. With less water lost to the atmosphere, more is available to recharge the groundwater, he said.
Schilling said that base flow — the portion of stream flow that is not runoff and results from seepage of water from the ground into a channel — has nearly doubled since 1940, largely because of the wholesale conversion of perennial vegetation to row crops.
The trend line for base flow as a percentage of stream flow has increased from about 31 percent in 1940 to more than 50 percent in 2000.
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With their elevated base flows, Iowa rivers, fueled by increasingly frequent heavy rains, can quickly rise above suitable recreation levels and stay above those levels longer.
My threshold for fishing the Wapsi in my waders is 5 feet on the gauge at Independence. At that level I can cross the river almost anywhere, wade comfortably upstream or down and effectively present lures as I go.
Boat and shore anglers, as well as kayakers and canoeists, can tolerate higher water and still enjoy their recreation, but they too have experienced more frequent and lengthy high water interruptions.
As a wading angler, I always thought it would be nice to be 7 feet tall. After my NBA career, I would be able to wade rivers a foot deeper than my current limitations, I thought. But of course that’s not true.
At 6 feet on the Independence gauge, the river is discharging 1,300 CFS — a 433 percent increase over the 300 CFS discharge when the gauge reads 5 feet.
That increased discharge is accompanied by an increase in current velocity, which renders wading unsafe and angling ineffective.