By John Dodge

CHICAGO (CBS) — Heavy, persistent rain has led to a huge increase in the flow of streams across Chicagoland and nearly all of them are way above normal for this time of year, according to a survey by the U.S. Geological Survey.

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Stream flow is the volume of water that moves over a designated point over a fixed time period. The main influence on stream flow is precipitation run-off in the watershed.

Most streams and rivers in the area are elevated but not near flood stage, but the flow of water is much faster.

In many cases, the flows are 400 to 1,000 percent above the median for that particular body of water.

For example, on Thursday the Hart Ditch in Munster, Ind., was flowing at 204 cubic feet per second, which is a whopping 971 percent above its median.

The North Branch of the Chicago River in Northbrook, which is a relatively slow-moving body of water, was moving at 27 cubic feet per second, 444 percent above the median.

Meantime the Des Plaines River in Riverside is moving along at 1,570 cubic feet per second, 491 percent above the median

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You can find other stream floor calculations here.

Why is this important?

Faster moving streams can cause more erosion.

The USGS says it affects water quality and the living organisms in streams.

High stream flows help remove pollution discharge from small streams.

Stream velocity determines the kinds of organisms that can live in the stream–some need fast-flowing areas; others need quiet waters.
It also affects the amount of silt and sediment carried by the stream.

Sediment introduced to quiet, slow-flowing streams will settle quickly to the stream bottom. Fast moving streams will keep sediment suspended longer in the water.

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Fast-moving streams also generally have higher levels of dissolved oxygen.