Operators reduced flooding in face of record-breaking year

As the sun sets on the runoff season, we have plenty of new entries for the record books.

One of the most impressive: The flows in the Colorado River above Lake Granby from April through July produced 429,000 acre feet, far surpassing the previous all-time record from 1984, which was 355,000 acre feet.

Lake Granby spillway releases on June 22

Although it's been a great year for rebuilding our reserves, we aren't always thinking about C-BT solely in terms of storage. We also know that huge runoff numbers raise huge concerns for people who live downstream of Lake Granby.

Although C-BT is not a flood control project, we were able to operate it this year in a way that prevented damaging flows. The pre-emptive releases operators made kept the releases downstream at or below about 2,300 cfs. If we hadn't tweaked operations to create these sustained lower flows, the downstream channel would have at times quickly risen to about 3,900 cfs.

For more details on how Northern Water and Reclamation worked together to reduce flooding, read the recent Sky-Hi Daily News guest column by Northern Water General Manager Eric Wilkinson.

You won't see too many entries to this online update for a while, but as always, we're keeping an eye on what to expect during the next runoff season. Given the bounty of this year's snowpack, we can tell you that we don't expect it to be a low-flow kind of year ...

Granby releases decreasing starting today

Our “second spill” from Lake Granby will be drawing to a close in the coming days. Last week’s monsoonal storms created some significant inflows to the system, causing us to increase and prolong our spill from what had been expected.

The diurnal (daily) snowmelt patterns are fading away. Notice on the North Inlet gage that the cyclic pattern is starting to flatten out. That means that the snow is nearly gone.

We will hold Lake Granby at elevation 8278.50 feet (that’s one foot below the operational full level) for a few days longer, just to ensure that we have space to store another rain event. After all, the monsoon pattern could shift back to the west in the coming days and weeks.



2011 spill releases from Lake Granby



Granby inflow beats all-time record

If you're into runoff-related recordkeeping, here's another one for the books. As of yesterday the total virgin inflow to Lake Granby was 379,000 acre feet. We expect that by the end of the month that number will top 400,000 acre feet.

The previous record was in 1984, with 355,000 acre feet coming in between April 1 and July 31.

Inflows to Willow Creek Reservoir are still running above average for this time of year, although we are noticing that the diurnal (daily) fluctuations are nearly gone. This is a sign that melting snow is no longer dominating the streamflow hydrology.

Increased releases from Lake Granby

A series of rain events this week has filled Lake Granby faster than we would have liked. Therefore, as of this morning, releases from Lake Granby have been increased from 1,900 cfs to 2,200 cfs.

We will stay at this release rate through the weekend and re-evaluate on Monday. More precipitation could change the plan, so we’ll be watching the skies. We still have space in the reservoir, but we want to preserve as much space as we can in the event that Mother Nature continues to pile on more precipitation.

Changes at WIllow Creek Reservoir and Lake Granby

Northern Water has determined it is time to initiate the second spill operation this season at Lake Granby. Outlet releases are being increased throughout today until outflows reach 1,500 cfs. We will maintain this release until at least next Monday. We are watching the skies this week – rainstorms are in the forecast and a significant rainfall event might prompt us to boost outflows even more.

At Willow Creek Reservoir, our West Slope operators will allow the reservoir elevation to rise to 8,129 feet this week. The high runoff this year generated a lot of debris which is now floating on the surface of the reservoir. The operators will adjust both the river outlet works and the gates on the pump canal. Prevailing winds should then help force the debris to move to the northeast corner of the reservoir where it can be removed. Once this operation is complete, the reservoir will be returned to its normal operating elevation of 8124 feet.

Granby Dam spillway water to taper off

We're continuing to reduce releases from Lake Granby this week. By Friday, water will no longer be going over the spillway; releases will only be coming out of the valves, which is where we normally make releases. We'll have 430 cfs coming out of the valves until re-evaluating the situation next week.

This spill was successful because it evacuated enough water so we could capture the peak inflow to the reservoir.

We expect to have a second spill as the reservoir approaches full capacity. It will be shorter and involve less water over the spillway. Operating the dam this way will allow us to gently fill the reservoir to the top while preventing the need to make large releases below the dam.

Granby outflows to decrease, elevation will rise

Engineers and operators have determined that an adequate amount of water has been released from Lake Granby, so we'll get back to filling the reservoir. We've kept the reservoir at lower levels to leave room for the finale of our forecasted runoff and to prevent the need for sudden large releases downstream.

We are going to decrease releases gradually, which limits environmental disruptions. We will cut flows back 200 cfs per day until we reach an outflow of 1,000 cfs, which will be early next week. Then we'll determine whether outflows should be reduced more.

With this reduction in releases, Lake Granby's elevation should begin to rise noticeably in the next week.

Willow Creek inflow record shattered, with more to come

After 57 years of tracking Willow Creek Reservoir stats, we've now hit an amazing new inflow record.

Some background: in the Western U.S., April through July are considered the snowmelt months, so official streamflow forecasts (and thus runoff records) are based on that four-month period.


Willow Creek Reservoir

Until now, Willow Creek Reservoir's inflow record was in 1986, when the April through July runoff totaled 85,600 acre feet, which is 168 percent of the official average of 50,800 acre feet. That sounded like a lot until yesterday, when we tabulated the inflow since April 1 as 87,100 acre feet. And we still have six weeks to go! 

Our latest forecast anticipated a total of 110,000 acre feet of inflow to Willow Creek by July 31. If it materializes, this would be 216 percent of the official average. And the Colorado Basin River Forecast Center expects even more than we do. 

The peak inflow rate (in daily average) this year was 1,652 cfs on June 7. This wasn't quite a record, but anything more than 1,500 cfs is notable for Willow Creek. This date of peak inflow was about two weeks later than usual (about May 24); late peaks often come hand-in-hand with larger-than-normal snowpacks.

This staggering runoff quantity raises questions about what kind of runoff we should expect from neighboring watersheds - especially above Lake Granby. While we don't expect as high an inflow to Granby, our latest forecast anticipates 175 percent of average, which would be another one for the record books.

It's disconcerting to drive past Granby, which is 20 feet below full, and see the surrounding dry sagebrush - if you know how high snowpack is in Rocky Mountain National Park and along the Gore Range.

That snowpack may seem remote and unassuming, but looks are deceiving. Stay tuned.

Granby releases to increase today

Spillway at Lake Granby as seen from below

The elevation of Lake Granby is now above the spillway elevation of 8,260 feet. Currently, the radial gates at the top of the spillway are closed, preventing water from flowing down the spillway. This morning, Northern Water’s West Slope operators will open the gates and water will start flowing down the spillway for the first time in 11 years. At first, the outflows will be rather small, on the order of 100 cfs or less. But as Lake Granby continues to fill, more and more water will flow down the spillway.
The increases in outflows will be evident at the downstream USGS gage.

Note that releases from the outlet valves will continue at approximately 410 cfs. The combined release from the valves and from the spillway will be approximately 500 cfs.

Warm weather triggers increased snowmelt

Warm weekend temperatures increased the inflow to Willow Creek Reservoir. Right now we are holding the reservoir's elevation steady at around 8,092 feet and passing inflows right through. The highest release so far this year was 782 cfs at 9:45 this morning.

Releases from Lake Granby are holding steady at about 400 cfs. Below the dam at Lake Granby there are small side tributaries that add to the flow of the river, so flows can be higher by the time they reach the "Y Gage" (listed as "Colorado River near Granby, CO" on the USGS website). Saturday evening was a good example: the flows were at 600 cfs, so the tributaries likely contributed about 200 cfs. The flow has not been as high since then.

The schematic shown with this post shows the various stream gages below Willow Creek Reservoir and Lake Granby. The releases from Granby and Willow Creek reservoirs, along with the flow of the Fraser River, all add to the Colorado River's flow as measured below Windy Gap Reservoir. Today, that flow went above 2,000 cfs for the first time this year. The flows are affected by Denver Water's diversions; they aren't diverting much Fraser River water right now because it has been so cold. 

Granby releases increase Friday

Beginning at 8 a.m. Friday, outflow from Lake Granby will increase by 50 cfs every two hours until outflow reaches 430 cfs in the late afternoon.

The flow of 430 cfs will be maintained indefinitely. It is the maximum that operators can release out of the valves that sit toward the bottom of Granby. If more water needs to be released, it will come down the spillway at the top of the reservoir, but only after the reservoir rises to the spillway's elevation.

The Colorado Basin River Forecast Center is predicting inflow to Lake Granby to be the largest on record. Northern Water's models agree.

Lake Granby releases increase starting tonight

The April 1 forecasts (the latest available) are indicating a greater inflow to Lake Granby this year than forecasters previously expected. Accordingly, operators are going to make a pre-emptive release of stored water in Lake Granby. Pre-emptive releases help operators balance inflows into a reservoir with the flows going out, and they can curb the need to ramp up releases at faster rates.

Outflow from the reservoir will increase from the current 20 cfs to 80 cfs this evening. Additional increases are scheduled through this week until a rate of 200 cfs is reached; that rate will continue until further notice.

Background: Lake Granby

Lake Granby is the largest storage reservoir in the C-BT Project. It is located on the Colorado River about 4 1/2 miles northeast of Granby.

Water is pumped from the Farr Pump Plant at the north end of Lake Granby to Shadow Mountain Reservoir through the Granby Pump Canal. Pump head elevations vary depending upon the reservoir's surface elevation.

The first water storage at Lake Granby began on Sept. 14, 1949.