Pactola Reservoir is full — nearly as full as it has been since records were first kept in 1956.

It is so full of runoff from the October blizzard and other storms that the city is having to postpone for a year a $3.7 million project to rebuild the Canyon Lake dam.

The massive amount of water is due to the second greatest amount of water draining into Pactola Reservoir from the Black Hills since 1956 for the months of October and November.

Curt Anderson, a civil engineer with the Bureau of Reclamation that owns and manages the Black Hills' larger reservoirs, said on average about 2,000 acre-feet of water is expected to drain into the Pactola Reservoir during October, which is about 652 million gallons.

Pactola drains into Canyon Lake via Rapid Creek.

But this year, Anderson said, the reservoir was inundated with 7,900 acre-feet of water — about 2.5 billion gallons — during October.

Around 1.7 billion gallons also drained into Pactola during November, which is also about four times the November average.

"This is the second largest inflow in October for the period of records for the reservoir," said Anderson, adding that those records date back to 1956. "Our inflows for November were 5,400 acre-feet and the average for November is 1,500. They, too, are the second highest on record."

He said the record for both months was set in 1999, at 8,400 acre-feet in October and 8,300 acre-feet in November.

"That (October) storm, that was a big storm that occurred," Anderson said. "That just changed all the parameters."

Holding off on rebuilding the 40-year-old dam will cost the city an additional $33,299, according to the city.

"The work they had done, they had to clean up to make the park accessible again," Keith Johnson, the city's project administrator for the dam rebuild, said of the added cost. "We're looking at October 2014, with a June 2015 completion, basically moving it back one year."

City officials originally anticipated a June 2014 project completion.

Public Works Director Terry Wolterstorff said a 2009 study of the dam, which was last rebuilt shortly after the 1972 flood, revealed it needed about $2 million worth of repairs.

"There was some cracking and some minor settling that had occurred along that spillway," Wolterstorff said. "By the time you do the repairs to it, you might as well rebuild the whole structure.”

To complete the project, the city had to install a bypass pipe and a smaller temporary dam at the western end of Canyon Lake to divert water flowing in from Pactola Reservoir.

"We have to actually build a little coffer dam at the upper end of the lake that would divert (water) into that bypass pipe," Johnson said.

The pipe, however, can handle only about a third of the water now flowing into Canyon Lake from Pactola after the blizzard and other storms, which have saturated the region's drainage basins.

"Once the Hills get saturated like that, they just continue to run water, even after it's melted off," John Wagner, the city's Water Division superintendent, said. "There were a lot of streams running that hadn't run for a while."

To allow reconstruction of the Canyon Lake dam, water had to be drained from the Pactola Reservoir to limit the amount of water being released from the reservoir into Canyon Lake.

Anderson said Pactola had been drained about 7 feet to allow enough space for wintertime drainage from the Black Hills.

Draining the reservoir allowed the Bureau of Reclamation to start releasing 20 cubic feet per second (CFS) of water — about 150 gallons per second — from Pactola's spillway, which is just short of the 25 CFS average during the winter.

The 20 CFS being released into the lake allowed the city to divert the water into Rapid Creek with its bypass pipe and reconstruct the dam.

Given the surplus of water that drained into the reservoir in October and November, however, that could not be sustained.

"We had room for inflows but then we got that storm in the first part of October, and we could not limit release to 20 CFS because we would have gone into the flood pool," said Anderson, adding that would have posed a flood risk.

To keep the water at bay, the bureau had to increase the amount of water being released from the reservoir to 65 CFS, which is about 487 gallons per second and about three times what the city's bypass pipe at Canyon Lake could divert.

"We couldn't handle that much flow," Johnson said.

Contact John Lee McLaughlin at 394-8421 or john.mclaughlin@rapidcityjournal.com

(3) comments

Adam

The city could make an agreement to have Pactola reservoir hold extra water back, then drain Canyon lake completely. After drained, it could be dredged, deepened, all the trash could be cleaned out and then stone and sand could be placed around the edge. Doing this would make it a much more useful lake, and then the reservoir could make a large release in the spring filling the lake completely in a week. But I would hope a new dam would be built to last at least 100 years, if not 200. Sad if a dam only lasts 40 years. Could make it have the ability to produce power.

I don't know if many have noticed, the pond by the civic center was drained, but the trash and silt did not get removed to clean up the body of water. So sad to go to all that work and then not even bother to clean it up. I would hope that the city will not make the same mistake with Canyon Lake.

lol, that's just upstream from the Canyon Lake complex. I've biked by that on the bike trail.

I hope during the rebuild that they dredge the lake near the inlet, I can walk in the lake nearly to the bridge and not get wet above my thighs, and in the shallow area to the right near where the bird-house is, it's ankle to knee deep all the way across. I took photos with a yard-stick and sent them to Wooster when he still worked there, but nothing came of it... *shrugs*.

farmer

By the photo provided it looks like Canyon Lake Dam was made by Beavers. Funny..

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