Floodwater takes its time to reach rivers

Hydrologists say predicting the pace of downstream flooding on the Fox and Des Plaines rivers is tricky.

Rita Lee, a hydrologist with the Illinois Department of Natural Resources, said the Fox River is especially difficult to peg. When the Fox River crests in a flood in New Munster, Wis., it takes about 18 to 24 hours for the water to arrive in the Chain O' Lakes region, she said.

However, there's a delay in its southward movement from there, Lee said.

That's because Fox River flood water doesn't flow continuously. It typically gets trapped in the Chain before it proceeds downstream to McHenry and Algonquin.

The Des Plaines River behaves in a similar manner, National Weather Service hydrologist William Morris said.

When the Des Plaines River crests during a flood measured at the Russell station near the Wisconsin border, there is no orderly flow for the water.

The shape of the river basin, runoff factors and tributaries means flooding doesn't occur next in Gurnee, Morris said. Instead, it typically occurs farther downstream in Des Plaines.

"It's not a uniform channel," he said.