Scientists track effects of heavy rain on Tahoe clarity

Rainstorms, like the one that hammered the Lake Tahoe Basin this past week, mean different things to different people. It can be a welcome respite from the persistent dry summer weather or an annoying interference with outdoor recreation. Children find excitement, bus commuters huddle in the shelters and windshield wipers sellers see dollar signs.

But the water quality experts at the Tahoe Regional Planning Agency see heavy precipitation more seriously, as it often raises amount of harmful fine sediment flowing into Lake Tahoe.

During the past week's storm, erosion control workers assessed the effectiveness of lakewide Best Management Practices - measures on all developed properties at Lake Tahoe to capture fine sediment as close to the source as possible.

"We spend the entire year preparing property owners for rain, so it's always impressive to see the power of these storms and how our BMPs work," said TRPA Erosion Control Planner Jessica Schwing.

Wednesday morning Schwing braved the downpour to check on stormwater control projects. She visited near Rocky Point Road in South Lake Tahoe, where a neighborhood-wide curb, gutter and infiltration basin project completed last year is now treating runoff.

The city of South Lake Tahoe implemented the project in 2009 with additional funding from the California-Tahoe Conservancy and the Environmental Improvement Program, said TRPA Spokesman Jeff Cowen.

One thing TRPA and other water quality agencies assess is how residential BMPs along the street are affecting the curb and gutter project, Cowen said.

"See that?" Schwing said during the Wednesday storm while pointing to a small brown channel running from a residence into an overflowing gutter. "That's dragging extra sediment into the gutter, and when enough properties do that, they overload the drainage system further down. Essentially, it's the small homes that are the first line of defense during a storm."

Schwing then dipped a glass vile into the gutter a few doors down to test the clarity of the water for the turbity (the density of suspended particles). She plugged the vile into a digital meter and watched its display jump up until finally stopping at 219.

Water pulled straight out of Lake Tahoe normally has a turbidity of 1, and the regional standard for stormwater discharge is 20, Schwing said.

Driving around neighborhoods, the conditions varied broadly. Driveways with perfect BMPs were functioning nicely next to mud-seeping, dirt parking areas, Schwing said. At one street intersection, a storm drain bearing a medallion that read, "Drains Directly to Lake" was surrounded by fan-shaped piles of fine soil.

"During heavy storms, developed parcels where vehicles drive or park on bare soils discharge sediment directly onto the City streets and into City storm drain systems," said Robert Erlich, stormwater coordinator for the City of South Lake Tahoe. "This extra dose of sediment and storm runoff from properties without effective BMPs makes additional work for street maintenance crews trying to keep storm drains open and streets clear of debris during the storm. It also adds to the city's long-term costs to clean the storm drain system and infiltration facilities after the storm."

Sediment-heavy runoff is not exclusive to steep areas where runoff potential is highest, Cowen said.

"Even streets with almost zero slope were lined with fast-moving channels of water during surges in the storm," he said.

More than 50 agencies are involved in the restoration of Lake Tahoe, Cowen said. Monitoring BMP effectiveness is the job of the project administrator, and in the case of Rocky Point, oversight fell to the City of South Lake Tahoe.

TRPA's Erosion Control Team and many project partners will use visual assessments taken from this storm system to help partner agencies and homeowners understand the impact of residential BMPs on neighboring stormwater projects, Cowen said.