Wisconsin River yawned but did not awaken

 

The 70-degree air that melted the snow in the valley March 15-18 elicited a yawn from the Wisconsin River but did not awaken the 430-mile long stream from its winter slumber according to Wisconsin Valley Improvement Company (WVIC), Wausau.

 

WVIC operates 21 headwater storage reservoirs for water conservation, flood control, streamflow regulation and increased hydropower generation under license from the Federal Energy Regulatory Commission.

 

A storm swirled over the Great Plains March 19-20 but produced less than a half inch of rain.  With only snowmelt runoff to encourage its arousal, the state's largest river stirred, but its rise in flow was a quarter of a 2-year flood, a high flow that statistically occurs once every two years.  It has been a yawner of a spring break up, so far.

 

 

Break up was subdued because the supply of water available was a below normal.  The water content of upper basin snow was half of normal.  Less than an inch of rain fell.  The runoff that occurred was captured in reservoirs and not released to the mainstem.

 

Large flowages on the river reduced the flood peak.  For example, the 10,523 cubic feet per second (cfs) 24-hour average flow at Nekoosa March 20 became 5,132 cfs below Castle Rock dam.  The intervening 21,000-acre Petenwell and 16,000-acre Castle Rock flowages were being refilled to normal operating levels with the excess flow.  Less water flowed out than had flowed in.  For contrast, a 2-year flood at Wisconsin Dells below Castle Rock is 34,000 cfs.

 

Snowmelt runoff stored in reservoirs

 

Time of concentration is a hydrology term that describes the period from when rain falls or snow melts and runoff reaches a river.

 

Along the Big Eau Pleine River and ending at the 7,000-acre Eau Pleine reservoir's confluence with the Wisconsin River in Marathon County, the time of concentration was fairly rapid.  After reaching a low of 25 percent of storage March 13, the reservoir filled to 46 percent of storage by March 20.

 

 

On the forested basin in Vilas, Oneida and Lincoln counties, the time of concentration will be longer.  The wooded, wetland-pocked watershed releases runoff slowly, vegetation and topography naturally attenuating or reducing the flood.  The rise in the amount of water in storage in the 16 natural-lake and four man-made reservoirs will be more gradual as a result.

 

Having been compiled before breakup, the March 16 reservoir report, showing total storage at 21.03 percent, will have marked the lowest level of storage during, and the end of the 2002-2003 April-March reservoir year.  WVIC had forecast a carry over of some 20 percent of water in storage as a conservation hedge against below normal precipitation during the winter.

 

 

Flow rates on central basin increased the most

 

On average, temperatures during the March 15-18 warm up were cooler over the upper Wisconsin basin because evaporation of a deeper snow layer chilled the air.  Bare ground on the central and lower basin absorbed more solar heating and high temperatures broke 70 degrees.  Melting there was more rapid and generated the greatest volumes of runoff and subsequent rises in Wisconsin River weekly average regulated flow rates March 14-20.

 

 

Spring equinox arrives cloudy, cool and wet

 

Rain March 19-20 that spun into Wisconsin from the cut off low swirling over the Great Plains fell in greater amounts in the central and lower Wisconsin Valley than in the north.  Rain and snow were forecast to continue nearly daily through March 29.  The start of spring March 20 at 7:00 p.m. central time was going to be cloudy, cool and wet.