Two minor mudslides occurred on the Island last week as the heavy rains brought flooded basements and unprecedented flows into the city’s stormwater system, polluting Lake Washington and local streams and ponds.

The heavy rains caused several private drainage systems to be overwhelmed. City maintenance crews provided sand bags for three Island homes to keep water away from flooding garages and foundations. Crews worked on the Island from about midnight, Sunday, until 11 p.m. the next day, receiving more than 60 calls from residents. That is about 10 times more than a typical winter day.

The city’s development services department also investigated three possible mudslides and confirmed that two occurred on the Island last week. A third incident involved a ground shift of about one foot but did not slide further. In all three cases, a geotechnical engineer was contacted to further examine the stability of those areas.

The county is also recommending that Islanders and their pets avoid contact with the waters of Lake Washington because of unseasonal algae blooms.

The region’s mild weather so far this fall has expanded the algae-growing season in Lake Washington, according to a press release from the county’s Lakes Stewardship Program.

Sally Abella, a senior engineer the stewardship program, said that concentrated amounts of the algae have been found in sheltered areas of Lake Washington, Lake Union and the Lake Washington Ship Canal, where they collect after being wind-driven across expanses of water.

“This is a natural occurrence,” she said. “We don’t think it’s especially toxic or dangerous, but to be on the safe side, we’re recommending that people and pets stay away from visible concentrations of this algae.”

Samples tested by a county laboratory identified the algae as a blue-green species that forms little clumps and looks like grass blades that stick together and float.

In addition to the presence of the potentially harmful algae in the lake, wastewater also spilled from a manhole cover in the North end of the Island and highly diluted wastewater drained into Lake Washington.

According to Annie Kolb-Nelson, a spokesperson for King County, an estimated 10,000 to 50,000 gallons of mixed storm and wastewater overflowed down 97th Avenue S.E. from the manhole just after the intersection at S.E. 34th Street. The park, Fruitland Landing, was already closed to the public for restoration.

The county notified the city of the spill and plans to meet with the city staff next week to debrief and discuss plans for any needed restoration. The beach remains closed, as it was before the storm.

Initial reports also indicated another spill may have occurred at the county’s South end pump station, but Monica Van der Vieren, a water quality planner, said the South Mercer Pump Station is fine.

“It turned out that we had an overflow on the South Mercer Pump Station Force Main, which is actually in Renton, not on Mercer Island,” she said.

The spill is unrelated to the repairs that the county performed last winter and spring near the community center. Van der Vieren said that part of the system functioned as it should, even under the extreme conditions.

The fire and police departments also received more calls than normal during the rains. The Fire Department responded to 14 calls in total, over twice as many calls as is typical in a day, and the police responded to about 17 calls related to the heavy rains.