The city’s plan to remove much of the York Creek dam is raising issues such as environmental and habitat preservation, St. Helena’s water shortage, and the condition of the city’s lower reservoir.

The city’s goals are to remove the obstruction the dam poses to fish passage and prevent the environmental damage that would result if the built-up sediment behind the dam is released. In 1992, silted water was accidentally released during dam maintenance, resulting in a massive fish kill.

About 40,000 cubic yards of accumulated sediment will have to be removed. Much of that material will be deposited at Spring Mountain Vineyards and Clover Flat Landfill, but the city’s plan to deposit as much as 26,000 cubic yards of that material along the edges of the lower reservoir has neighbors worried about the impact on the reservoir’s capacity and habitat.

They were also worried about the possibility of, as Duane Hall said, “chunks of rebar and concrete sticking out of a reservoir that’s drained lower than normal.”

Public Works Director Jonathon Goldman tried to allay those fears, saying only dirt and rock material would be trucked to the lower reservoir. Current plans call for the York Creek dam’s concrete material to stay there, he said.

The lower reservoir is at its lowest point in memory, for several reasons: an ongoing drought, the lack of water being diverted to the reservoir from York Creek, and the city’s decision to draw down the dam last year to make way for geotechnical work in preparation for the dam removal project.

Aside from their concerns about the reservoir’s appearance — “it looks like a mudhole,” said Clare Kirkconnell Barr — neighbors are worried that depositing material at the reservoir will decrease the amount of water available for regular consumption and firefighting, and  will harm the local habitat.

“When all this was decided, was Fish and Game made aware of how one habitat would be suffering to help another habitat?” asked Barr.

The project includes a revegetation plan for the affected areas of the lower reservoir. Goldman said the effect on the reservoir’s storage capacity will be minimal. According to the project’s EIR, the reservoir’s capacity could be reduced by as much as 3 percent.

The Department of Fish and Game forced the city to stop diverting water from York Creek in 1992. Resuming the diversion would require the city to build a new structure, priced at about $500,000, to allow fish passage.

In addition, Goldman said, the structure’s usefulness would be limited by Fish and Game restrictions, which would only allow diversion when creek flows reach a high volume — and at those times, the lower reservoir generally fills up on its own. In drier-than-average rainy seasons such as the last one, diversion would be prohibited.

Diversion debated

Goldman said he wasn’t recommending building the diversion structure because the city’s limited resources would be better spent increasing the capacity of Bell Canyon reservoir, buying more water from the city of Napa, improving the reliability of the Rutherford pumping station that supplies water from Napa, or even rebuilding the lower reservoir’s antiquated dam to increase its capacity.

However, neighbors said they would like to see the new diversion structure incorporated into the dam removal project.

“We have an opportunity here to capture more water here,” said Hall. “But it looks like we’re going to bypass that to save a half-million dollars.”

According to the project’s EIR, another potential impact is an increase in the frequency and severity of flooding in lower York Creek below Highway 29 due to sediment accumulation. But according to the project’s consultants, hydraulic modeling predicted a neutral impact on flooding.

The project hasn’t been completely designed, but early estimates put its cost at $7.4 million, split between the city and the U.S. Army Corps of Engineers. If approved by the council, work will begin next year, and continue for another two or three years.