January 23, 2003

As might be expected when the lowest temperatures of winter are supposed to occur, double digit below zero cold transformed a lot of water into ice on the Wisconsin River basin January 17-23. The solidification process consumed enough water to reduce flow in the 430-mile long stream according to Wisconsin Valley Improvement Company (WVIC), Wausau.

WVIC operates 21 headwater storage reservoirs to regulate a uniform flow in the Wisconsin River under license from the Federal Energy Regulatory Commission. Flow regulation increases power generation at 25 Tomahawk and Wisconsin River paper mill and utility hydroelectric dams.

Flow in the Wisconsin at Eagle River, for example, decreased from 530 cubic feet per second (cfs) Jan. 16 to 280 cfs Jan. 23. There were two reasons flow dropped. Cold changed water to ice and the release from eight natural-lake reservoirs above had begun to decrease as the levels in the lakes neared their winter minimums. Release from Burnt Rollways reservoir, the Three Lakes Chain of Lakes, was reduced Jan. 12-19 by 50 cfs, WVIC noted.

As the cold descended from the Arctic, advancing fronts on Jan. 18 and 19 wrung but a dusting of snow from the dry air both days where the changes in temperature occurred. January month-to-date precipitation continued at below normal amounts at Wisconsin Valley reporting stations.

One of only two locations on the Wisconsin where weekly average regulated flow was normal Jan. 17-23 was Merrill, a gaging station below 20 of WVIC's 21 reservoirs. WVIC is operating the reservoirs to achieve target flows of 2,000 cfs at Merrill and 2,900 cfs at Wisconsin Rapids.

Petenwell dam in Adams and Juneau counties was the other location where flow was normal. The elevation of the flowage above the hydroelectric dam in is being reduced to protect the shore from ice movement and to create storage capacity for spring flood flows. With more water being run through the turbines than is coming into the flowage, the level gradually drops and the resulting flow below was 102 percent of normal.

As a result of the loss to ice, natural flow in the Wisconsin River at Merrill, the flow that occurs without reservoir operation, was 1,096 cfs January 19, a decrease of 104 cfs. Natural flow was 81 percent of or 249 cfs below the 1,345 cfs January normal.

The shape of a reservoir, wide at the top and narrow at the bottom, influences the rate of increase or decrease in the percentage of water in storage. At this point in the winter release cycle, when the level of the water in a reservoir is several feet below maximum, the percentage of water in storage decreases at a faster rate as the size of the reservoir decreases because of its shape.

WVIC's five man-made reservoirs range in surface area from the Spirit's low of 1,700 acres to the Eau Pleine's high of 6,800 acres. The changes in water levels, determined by the authorized maximum and minimum elevations, range from the low of 15 feet at Rice to the high of 27 feet at the Eau Pleine.

Among the man-made reservoirs on Jan. 23, the average water level was 6.54 feet below maximum and the average percentage of storage was 46 percent.

Were they the coldest temperatures this winter? Wisconsin Valley low temperatures by location reported to the National Weather Service Jan. 23 included -15 Land 'O Lakes, -11 Eagle River, -11 Woodruff, -13 Willow Reservoir, -14 Rhinelander, -15 Tomahawk, -15 Medford, -13 Antigo, -12 Wausau, -13 Marshfield, -9 Stevens Point, -12 Wisconsin Rapids, -13 Friendship, -7 Lone Rock, -8 Boscobel, and -8 Prairie du Chien.

Weekly average temperatures at Willow reservoir in Oneida County and Wausau were eight and 14 degrees below normal, respectively.

Whether or not the Jan. 23 temperatures were the coldest of the winter won't be known until after Jan. 27. The National Weather Service had forecast deep cold again Jan. 23-24 and 25-26. Temperatures were forecast to moderate thereafter. A corresponding rise in the percentage for precipitation, however, did not accompany the forecast rise in temperature.

-End of Report-