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Delavan Lake still needs a little TLC

--- Lake could deteriorate without intervention, expert says

Mike Heine/The Week

(Published Oct. 15, 2007, 2:21 p.m.)

Dr. Dale Robertson, a research hydrologist with the United States Geological Survey, is worried about Delavan Lake.

He has been studying it since 1993, shortly after millions were sunk into the 2,000-acre-plus body of water to eliminate a rough-fish population and stop phosphorus-induced algae blooms that made enjoying the lake for recreation virtually impossible.

Without continued work, the lake could again become a turbid body of muddy green water filled with carp and bigmouth buffalo, Robertson said.

"If you don't eliminate the source of the problem, the phosphorus loading, things are going to get worse," Robertson said at a Delavan Lake Committee meeting recently.

Phosphorus, a chemical found in natural (manure) and man-made fertilizers, helps the crops and lawns stay green in summer. It also promotes weed and algae growth in lakes when introduced through runoff, he said.

In the '70s, Delavan Lake was considered to have the second-worst lake ecology in the state, Robertson said. Phosphorus poured in from internal and external sources at more than 14,000 kilograms per year.

Since then, a slew of new practices were tried to reduce the phosphorus coming into the lake and balance the ecology to improve the fishery and water clarity.

Efforts starting in 1989 included:

--- Construction of three retention ponds in wetlands areas above stream from the Delavan Lake inlet to act as filters for water coming in from Jackson Creek in Elkhorn.

--- Construction of a large rock peninsula to try to divert water coming in from the inlet directly toward the outlet so excess nutrients wouldn't permeate through the main lake.

--- A lake drawdown and poisoning of all fish remaining in the lake after game fish were netted out. The effort was to kill a dominant rough fish population.

--- Game fish were restocked to create a balanced fishery devoid of rough fish.

--- Modifications at the outlet dam to keep fish from re-entering the lake.

--- Applying a chemical treatment of alum (aluminum sulfate) to the bottom of the lake to "seal off" sediment and prevent nutrient loading from within the lake itself.

Years prior to that, Elkhorn constructed a sewage treatment plan and the Delavan Lake Sanitary District installed a sewer system for homes near the lake, removing septic systems.

Also, best management farm techniques were promoted throughout the 27,000-acre watershed and the town recently instituted a ban of phosphorus-based lawn fertilizers, Robertson said.

Has it worked?

Most everything that was done for the lake has showed some measure of success, but not to the levels planners had hoped, Robertson said.

The water clarity is well past the goal of 1.5 meters, but the phosphorus levels coming into the lake are above goals. The filtering ponds trapped about 30 percent of the phosphorus, less than half the anticipated amount.

It appears the alum treatment has been covered over by new sediment and phosphorus is again coming up from the lake bottom, Robertson said.

The lake would likely be greener if it weren't for a balanced fishery, the most successful part of the project, Robertson said.

"We think it goes back to the bio manipulation," he said. "That is what has kept the lake and it's still having an effect on the lake today."

But even that is seeing signs of trouble with carp and bullhead being seen and caught with more regularity.

"It's never going to get back to the way it was in the '70s. It's not going to get that bad," Robertson said. But, "It could continue to degenerate. It could get bad. Carp could take over slowly. They stir up the weeds and it could become a mess."

What next?

"That's hard to say," Robertson said. "The success of this particular project depends on what you're looking at. If you're interested in water clarity and fish populations, we're still in good shape and are much better than what we were hoping. The fish population, if you talk to the DNR, they're ecstatic. (But) the algae blooms are bad."

The town has already taken a major step by starting to dredge the retention ponds at the inlet, deepening them so they trap more nutrients and sediment. The town is planning to redesign the ponds so they become more effective, Robertson said.

The town could explore another alum treatment, but the cost-benefit factors over time are yet unknown, Robertson said. Excessive rains in 1993 essentially washed out the initial treatment and that could happen again.

The fishery is still in good shape and it is hoped the game fish population will control the rough fish. That situation needs continuous monitoring, Robertson said.

Another important element to lake health is to monitor urban development and make sure it's done right to reduce runoff from entering the lake.

"Agriculture has 10 times as many nutrients as a forest," Robertson said. "Hands down, that's where most of it is coming from. As (the area) gets more urbanized, it has to be done in the right way. If urban areas go up in the wrong way, it can be much, much worse than agriculture."

The message though is clear. The project definitely isn't over, Robertson said.

"Depending on who you talk to, this is a huge success. Maybe it won't last forever," he said.



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