What really is the crest? Flooding terminology made easy

  • The Algonquin dam is barely distinguishable as the rising Fox River neared its crest Monday afternoon.

      The Algonquin dam is barely distinguishable as the rising Fox River neared its crest Monday afternoon. Rick West | Staff Photographer

Updated 7/17/2017 5:11 PM

Flooding can be confusing.

With terms like "river crest," "datum points," and "CFS" being thrown around, figuring out how high the water will rise in your crawl space and when can be difficult.


Thankfully, Rita Lee, the engineering studies section chief from the Illinois Department of Natural Resources Office of Water Resources, took the time to develop easy-to-understand definitions for the most complex terms and phrases in flooding.

Here are some common terms used to describe flooding on the Des Plaines, Fox River and Chain O' Lakes and what they mean:

CFS: Short for cubic feet per second. This is the volume of water flowing. It tells officials from the Illinois Department of Natural Resources how full a lake or river will get. For example, if the Fox River is running into the Chain O' Lakes at 14,000 CFS, but leaving the Chain at 9,000 CFS, that means the lake will continue to fill because more water is coming in than going out. The Chain will crest when the inflows and outflows are the same, and it will lower when the outflow is greater than the inflow.

Crest: A term used to explain a water level that has reached the highest point it is expected to go. After a river or lake crests, the water will begin to decrease in the coming days.

Datum point: Possibly the most difficult term to understand. Due to varying heights of river and lake bottoms, the United States Geological Survey and the National Weather Service use a datum point to measure the heights of rivers. So, when you hear that a river crested at 14.5 feet, it does not mean the river is 14.5 feet deep. Instead it means the river is 14.5 feet above the datum point established by the USGS.

River gates: These are the "water doors" at both the McHenry Lock and Dam, and the Algonquin Dam. When the gates are open, the river is allowed to move freely through the dams down river. When the gates are closed, water is held back and not released south. The gates in McHenry and Algonquin have been running wide open since more than 7 inches of rain fell over the area last week.

Watershed: All land surrounding rivers and lakes belong to specific watersheds, and all rain that falls into a watershed runs into a specific river. The Fox River and Chain O' Lakes belong to the Fox River watershed, a 202-mile-long tributary of the Illinois River that flows from southeastern Wisconsin to Ottawa, Illinois. The Des Plaines River watershed flows south for 133 miles through southern Wisconsin and northern Illinois, eventually meeting the Kankakee River west of Channahon and from there into the Illinois River. The Illinois River serves as a tributary to the Mississippi River.

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