Runoff News: Colorado-Big Thompson, Aspinall Unit update

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From email from Reclamation (Kara Lamb):

This week, due to cooler temperatures, melting snow down the Big Thompson River into Lake Estes dropped off considerably. While Lake Estes is filled primarily by C-BT water coming from the west slope and delivered through the Estes Power Plant, we have been moving that water on down through the project and into Horsetooth Reservoir. When the natural inflow from Big Thompson dropped off, that dropped the water level of Lake Estes down a full foot to around 7470–about five feet below full. We anticipate that over the weekend, we will be able to store a little more behind Olympus Dam and raise the water level of Lake Estes back up that foot to around 7471 or maybe even 7472.

Downstream of Lake Estes, the Big Thompson River through the canyon has been fluctuating, reflecting the inflows we have been seeing from the upper Big Thompson River to the reservoir. We have been passing matching inflows through Olympus Dam on down through the canyon. The changing temperatures play the largest role: when it is warmer, more snow melts during the day, causing flows in the Big Thompson to rise at night. We adjust the gate at Olympus Dam to reflect those flows and pass them on down to the canyon. But, when it cools off, the inverse happens, less snow melts, and flows in the river drop off. That is why the Big Thompson below Olympus Dam started at around 175 on Monday, bumped up to near 300 cfs on Tuesday, dropped to just under 200 cfs on Wednesday, and is now flowing around 125 cfs. Depending on what the weather does, we will adjust the release from Olympus Dam at night to match the snowmelt coming down the river.

“Down canal” from Lake Estes is Pinewood Reservoir. Pinewood stores water above Flatiron Power Plant before we drop the water down the penstocks to generate hydro-electric power at the plant. We have been doing some upgrades at Flatiron for several months–and that will continue through the summer. Because of that, we are generating with one unit instead of two. This means, we cannot run as much water through the plant. So, we are not filling Pinewood as full as it typically has been this time of year in other years. As a result, Pinewood has been holding a fairly steady water elevation of around 6567 most of the spring. On Wednesday, the change in inflow at Lake Estes also effected Pinewood and the water elevation dropped three feet to about 6564–about 16 feet down from full. As we move through the weekend, the elevation at Pinewood will climb back a little bit, but we do not anticipate it getting much higher than 6570–ten feet down from full.

Carter Lake has remained steady at an elevation of 5753–about six feet down from full. Once warmer weather hits, we anticipate water users will begin to pull their water from Carter Lake. But, the rain has staved that off a bit. We anticipate Carter will maintain this water elevation through the weekend.

Similarly, Horsetooth Reservoir has been holding at a steady elevation of about 5416, almost 5417. Like Carter, we have not seen much water go out of Horsetooth, yet, for water users. Again, the rain is probably holding that off. We have been bringing only a little water into Horsetooth Reservoir over the past week because water users downstream on the Big Thompson River are taking some of their C-BT water. If that demand drops off and the cool weather continues, Horsetooth will rise slightly. Otherwise, it is likely to maintain the same water elevation through the weekend, as well.

From email from Reclamation (Dan Crabtree):

Blue Mesa Reservoir has been increasing in elevation at a rate of about 1 ft per day and the Colorado Basin River Forecast Center tells us the April – July inflow volume is likely to increase to over 700,000 ac-ft. To accommodate this change in inflow, releases from the Aspinall Unit will increase by 200 cfs on May 29th. Following this change, flows in the Black Canyon and Gunnison Gorge will be about 2,100 cfs.

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