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.

Willow Creek Reservoir elevation: Back to normal

After a couple of months of looking pretty low, Willow Creek Reservoir's elevation will start looking up.

The reservoir's elevation will start to rise in the next few days as we go back to more normal operations for this time of year. The elevation has been lower than the seasonal average because of releases, which since April 1 have equaled more than 106,300 acre feet - nearly two and a half times the amount that normally flows in.

In a typical year, Willow Creek's pump plant moves reservoir water to Lake Granby. But in this wet year, we knew Lake Granby wouldn't have room for that water, so we let it simply pass through Willow Creek Reservoir. There was so much Willow Creek water passing through that we had to keep the reservoir elevation much lower than usual so we could be prepared for heavier runoff peaks and help protect the channel downstream.

Now that the runoff is tapering, the risk of peaking events is significantly reduced and we feel comfortable letting Willow Creek Reservoir go back to its normal elevation. Over the next few days, it will gradually rise to 8,124 feet - about nine feet higher than it is today.

During this fill period, flows below the reservoir will be reduced, but once the reservoir reaches elevation, we will start passing inflows again.

This chart shows how Willow Creek inflows (red) compare to outflows (green) this year:



2011 Willow Creek Reservoir inflows (red) and outflows (green)