By Nathan Elder, water supply manager for Denver Water.

The Colorado River at Kremmling in Grand County will enjoy a big bump in flows from August into October as Denver Water pays off a hefty water debt. 

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The rising flows — an addition of more than 300 cubic feet per second (more on that later) sent from Wolford Mountain and Williams Fork reservoirs — serve as a good example of how Colorado’s intricate system of water rights can drive river flows higher when they might typically be lower as autumn settles in. 
  
In this case, it works like this: A dry year created conditions that now require Denver Water to “pay back” water to the West Slope. 
  
Why? Let’s stick with the easy version.
  
An agreement that emerged over 50 years of Byzantine legal fights allows Denver to move water from Dillon Reservoir in Summit County to the Front Range when it needs the water for its customers. 

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But — and this is a big “But” — if another big reservoir called Green Mountain (that’s the very long reservoir you drive past as you cruise Highway 9 between Silverthorne and Kremmling) — doesn’t fill up in the spring and summer, Denver Water has to make up the difference later in the year.

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Stay with us here. Take a look at the map that accompanies this story to help. 

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Years like this, when Denver Water has to refund water, are called “substitution” years. There have been big substitution years, when a lot of water is involved in the refund, in dry years such as 2002, 2004, 2012 and 2013. 
  
This year is shaping up as a big one, too; one of the largest. In all, the utility expects to release about 37,600 acre-feet from Williams Fork and Wolford to make up what Green Mountain, a reservoir operated by federal Bureau of Reclamation, lacked this year.
 
That’s a lot of water — close to the capacity of Gross Reservoir, the big Denver Water reservoir in the foothills northwest of Denver. An acre-foot is roughly enough water to serve three or four households for a year. 

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But wait, you say. Water from Williams Fork and Wolford won’t find its way to Green Mountain, since the Green Mountain Reservoir is on the Blue River and those two reservoirs send their water into the Colorado River, not the Blue. 

(Also, water can’t flow upstream from the Colorado River into Green Mountain Reservoir. Take another look at the map in this story.)
  
That’s OK, as the point is to make up for flows in the Colorado River that would otherwise be augmented by releases from Green Mountain. In short, the releases keep the flows moving on the West Slope. 
  
Now, back to those flows. Releases are expected to add an additional 400 cubic feet per second to the Colorado River in August, 320 cfs in September, and then decrease somewhat to an extra 200 cfs in the first two weeks of October. 

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How much water is that? 

Quite a bit. If you think in terms of gallons (think of the gallon of milk at the grocery story), a cubic foot contains about 7.5 gallons. So 300 cubic feet per second means about 2,250 gallons of water per second added to the river flows. (Think about that many milk jugs floating by each second). 
  
While it’s a lot of water to pay back — and it means Denver Water will need to draw down its supplies in Wolford and Williams Fork quite a bit — it could have been even more. 
  
But a wet spring on the Front Range kept sprinklers off and demand low. Monsoons returned this year as well, boosting flows on both sides of the Continental Divide. All of that allowed Denver Water to reduce what it moved from Dillon Reservoir, through the Roberts Tunnel, to the Front Range. 
  
Which, in turn, allowed a bit more water down the Blue River and into Green Mountain and reduced the “substitution” amount to be repaid. 

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If you’ve stuck with us until now, we raise a toast to you, salute your interest in a puzzling topic, and hope that this boost in late season flows in the Colorado River brings a smile to all of us inspired by the beauty of a moving stream.