DECORAH — I had a revelation Tuesday during a presentation at Luther College by Department of Natural Resources Northeast Iowa Fisheries Supervisor Mike Steuck.
Steuck, one of several speakers at a forum on “Karst, Water Quality and Livestock,” explained how karst topography shapes the trout streams of northeast Iowa.
It’s no coincidence, he said, that 22 of the 32 stream segments designated as outstanding waters by the state are in three counties — Allamakee, Winneshiek and Clayton.
Those counties, he said, are almost entirely underlain by shallow, easily fractured, soluble limestone and dolomite bedrock — the definition of karst.
The springs, caves, sinkholes and underground streams characteristic of karst provide the cool, clear, relatively pure water trout need to flourish, he said.
In many of those “outstanding waters,” trout swim in surface water that only moments before had been underground, largely free of the silt and other pollutants that run into streams after heavy rains.
While I had long been aware of the relationship between karst and trout, one of Steuck’s handouts helped me understand karst also contributes to the good fishing in many of the larger warm-water rivers of Eastern Iowa.
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I had mistakenly thought karst was largely confined to trout country, where caves, sinkholes and limestone outcroppings abound. But the DNR map showed areas of potential karst, defined as “carbonate bedrock within 50 feet of the surface,” along lengthy stretches of the Cedar, Shell Rock, Wapsipinicon, Maquoketa and other Eastern Iowa rivers.
My “aha moment” came when I realized that the areas of those streams that consistently provide the best fishing coincide closely with the prevalence of carbonate bedrock so close to the surface that you can trip over it.
Though I have always correlated the good fishing with the prevalence of rock on the banks and in the stream bed, it had never dawned on me that the rock could be an expression of karst.
Besides the fish-friendly in-stream habitat afforded by rock, Steuck said karst-associated streams are fed by a higher percentage of base flow — that portion of water that seeps slowly into the stream from groundwater.
Streams flowing through karst, he said, typically have better clarity and better water quality in general than streams fed primarily by runoff from agricultural fields.
While knowing the relationship between karst and some of my favorite river stretches won’t help me catch more fish, it makes me feel better knowing my long-standing ignorance did not prove invincible.