Most west-metro lakes were cleaner last year, but the improvement was more a product of the dry weather than efforts to curb pollution, the Minnehaha Creek Watershed District says.

Lower-than-usual rainfall caused fewer pollutants to wash into the lakes, so most improved by a half to a full letter grade in ratings just released by the district.

"There was 24 percent less rainfall and associated runoff contributing pollution to area lakes," the district noted in releasing the grades.

By the end of June last year, the creeks leading into Lake Minnetonka were barely flowing, and rainfall did not pick back up to normal levels through the fall, said Udai Singh, the Watershed District's water quality specialist.

Both "the timing of the rain and how much falls" will affect the amount of pollutants that wash into area lakes, Singh said. While the weather has been dry so far this year, it's too early to tell whether the trend is continuing.

The Watershed District has been grading its lakes on an A-to-F scale since 1989 based on water clarity, algae growth and phosphorus levels. Of the 62 sites graded in 2008, 40 showed improvement, nine were rated lower in quality, and 11 were unchanged from 2007. Two had not been monitored the previous year.

Lake Calhoun in Minneapolis improved from an A- in 2007 to an A in 2008, though nearby Lake Harriet was an exception to the rule: It dropped from an A- to a B+.

Lake Minnetonka's numerous bays and connecting lakes were graded individually, and 24 out of 26 showed improvement. Wayzata Bay and Grays Bay on the eastern end of Lake Minnetonka both moved up from a B to an A. On the western end of Lake Minnetonka, Cooks Bay on the shores of Mound improved from a C+ to a B.

"It's very good news," said Dick Osgood, executive director of the Lake Minnetonka Association. While acknowledging "blips we see year to year can be explained by lower or higher rainfall," he said it is clear "the longer-term trend has been positive" -- though he says the lake continues to face a severe threat from invasive species.

Beautiful to 'not enjoyable'

Lakes with an A grade are described as the top 10 percent in terms of water quality: "Crystal clear, beautiful" and suitable to be "enjoyed recreationally without question or hesitation." Lakes with a B grade fall in the next 20 percent, with "generally good water quality," though "algae may limit swimming, particularly toward the end of summer."

Lakes that previously scored in the lower range also generally showed improvement.

Long Lake got a C (up from a C-), indicating it is of "average quality" but its "swimming, boating and fishing may be undesirable relatively early in the season"; Halstead Bay south of Mound scored a D+ (up from a D) indicating "severe algae problems"; and Katrina Lake in Medina, Mud Lake near St. Bonifacius and Langdon Lake in Mound all rated F's, indicating they are "not enjoyable ... [with] severe limitations to recreational use."

But there were other lakes that continued to be stars in the ratings -- such as Christmas Lake, which has earned straight A's for the past decade, its sole blemish being an A- in 2004.

Ratings for all of the sites graded by the Watershed District can be found at .

On the website, the district lists the lakes' grades throughout this decade, and in most cases they show no more than a letter grade's variation, either from year to year or over the span of the decade.

That reflects the fact that changes are hard to come by when trying to improve water quality, Singh said.

"Our goal is not only to protect them and manage them but to improve them," Singh said of the area's lakes. But "water quality improvement is a long process. It took a long time to get these lakes where they are, and it will take a long time to get them back" to a better level of water quality.

In the meantime, while the more polluted lakes are not getting a lot better, they are at least not getting dramatically worse either, Singh noted.

Sam Barnes • 612-673-7840