By Tom P., October 8, 2021

Earlier this month a few members of various Trout Unlimited chapters in Colorado met for a tour of the proposed Wild Horse Reservoir. The tour and presentation were given by Aurora Water representatives at the proposed location in South Park. The representatives were very passionate and I learned a lot about the water issues and challenges that they face.

In a nutshell, 80% of Colorado’s population lives on the Front Range, and 80% of Colorado’s water is west of the Continental Divide. Cities like Denver, Aurora, and Colorado Springs have purchased water rights all over the state, and engineers have designed a complex water delivery system to move the water east. It is a really impressive system — water stored in reservoirs, piped up over mountain ranges, sent down natural rivers and creeks, then moved again. Apart from the dewatering impacts to the original watersheds, which is not my aim to discuss, we sometimes do get some benefit from some of that water flowing down the Arkansas river.

Water management is a very complicated process – while the demand side is fairly well known, the supply is extremely variable. I tried to explain it as best I could in layman terms, but that involved a lot of oversimplifying. Greg Baker from Aurora Water, one of the presenters on the tour, kindly made some comments clarifying some issues. I took the liberty to quote them verbatim, as to both have the proper explanation and to shed light on how complex this process is. Each quote is attributed and in italics for clarity.

Aurora Water's water supply chart
Map of the Aurora Water’s “plumbing” system

Some of the challenges that I found interesting:

– Aurora owns only 2,700 acre-feet (AF) of storage in Twin Lakes. That is not much, and Aurora moves 50,000 AF (!!) through that reservoir a year. That means that whatever water they get from the Colorado river watershed has to pretty much go right through Twin Lakes into the Arkansas. [Greg Baker: Water gets complicated. From Twin Lakes, our Colorado River water uses a pipeline to our pump station outside of Buena Vista. Here it’s moved into the S. Platte basin. Our Arkansas River rights, mostly from the Rocky Ford area, do impact the Ark since we move them from Pueblo Reservoir to Twin Lakes (via exchange, but that means water is moving down the river to meet the needs fo whoever we exchange with). We are part of the Browns Canyon Flow Program, which helps ensure minimum flows between Twin Lakes and Pueblo Reservoir from July 1 – August 15 to maintain a healthy fishery and help the recreational boating community.]

– They have limited amounts of storage in various reservoirs. If reservoir managers expect increased runoff and have to “make space” for it, they can force Aurora to move their water. If Aurora has limited amount of storage downstream, then sometimes the water has to be “spilled” — dumped into the river, flowing through other catchments, down into Kansas.

– The city of Aurora uses about 50,000 AF annually. Their total available storage, across the entire system, is 155,000 AF. In other words, if their system was filled to capacity, they would have 3 years of usage. This is not a long time, considering climate change, drought in the West, and the decreasing annual snowpack.

– Aurora can use an exchange system to trade water — since water from Pueblo Reservoir is hard to transport to Aurora, they can exchange it for water that’s at higher elevation closer to their “plumbing” system (e.g. to Twin Lakes or Turquoise). Quite a lot of this goes on, allowing water utilities to make use of water they have rights to but that is difficult to access.

– Water movement is pretty tightly regulated. The VFMP on the Arkansas dictates exactly how much water participants can move, and when, as to maximize benefits to the fishery, the rafting industry, agriculture, etc.

Aurora Water's storage and supply chart.
Chart showing Aurora’s water storage capacity vs. supply over time. Note the last 5 years — each peak is successively lower, reflecting the decreasing amount of runoff every year.

Currently, water from the Colorado river watershed moves through Twin Lakes, then down the Arkansas to about Rapid #5 on the Numbers section. [Greg Baker: We do have an alternative diversion at Granite, the Arkansas River Diversion, that we just rebuilt in cooperation with CPW, CWCB and others, where could divert water directly from the Ark, but that is a back-up diversion only. Due to the large amount of water that we and Colorado Springs Utilities move through the pump station, we prefer not to use the river diversion unless absolutely necessary].

The Otero Pump station is able to pump a huge amount of water over the mountains, moving it from our river to the South Park valley. Currently, that water runs through the Spinney channel and dumps directly into Spinney Reservoir. The channel is open to the sky, freezes in the winter, has evaporation issues in the summer. It runs about 200cfs, and moves about 150 AF annually. From Spinney, the water runs through the Dream Stream, Elevenmile Reservoir, and down through a network of other storage reservoirs.

Water from the South Platte drainage, which comes through the South, Middle, and North forks of the South Platte river, currently flows directly into Spinney Reservoir. I’m sure that many of us fished the various sections of those rivers in South Park.

Aurora has about 53,000 AF of storage in Spinney itself. One of the big challenges they face is that in years with a lot of runoff they get too much water in store in Spinney. Remember — water from both the Colorado and Arkansas river watersheds goes into Spinney as well. If the Platte river is coming in full, then Spinney has no space to store the water from the Colorado river. So all that water goes into Spinney and gets immediately spilled through the Dream Stream, going down the system, the water has to go down to Pueblo Reservoir in an unconstrained manner. [Greg Baker: Again, complex. If we see that we’ll have a huge yield in the S. Platte, we may delay pumping from Twin, but that means we have to release anything we can store in Homestake, Turquoise or Twin into the Ark in an unconstrained manner. This may result in too much flow that could impact the fishery in the spring, which impacts spawning. On the other hand, if we have a year like 2020-21, where we had a very late heavy snow ad rain event in the upper S. Platte, we have already filled Spinney with CO and Ark water, and then have figure what to do unanticipated Platte water. This impacts the Dream Stream and the Platte downstream of Eleven Mile (that’s a flood control reservoir, so they have to pass our excess flows right through].

Touring the site

To remedy this issue, Aurora is planning to build a new reservoir called Wild Horse. It would be located in South Park, west of Spinney. It would store exclusively the water that comes from the Colorado and Arkansas river watersheds, through the Otero pipeline. This reservoir would add about 96,000 AF of storage (remember: current total storage is 155,000 AF). The Wild Horse location is on private land, with some existing rock formations that make most of a natural bowl. Three dam sections would have to be constructed, and they would be a lot taller than Spinney’s. Maximum depth would be 197′, and it would have a much smaller surface area than most current reservoirs.

There is a lot less surface evaporation at higher elevations. It stands to reason — air is colder, water stays colder as well. But the numbers are staggering — it is estimated that annual evaporative loss at Wild Horse would be about 3′ a year (meaning the water level would drop 3′ annually). On the Front Range, which is lower in elevation and a lot warmer, that loss can be 45′!

Wild Horse reservoir would allow Aurora to store all the water that comes from the Colorado river and Arkansas watersheds, and send it on through Spinney as needed. It’s a natural place in the system to interrupt the current water flow, and it would allow Aurora to optimize the storage they have in Spinney. If a lot of water is coming down the Platte, they could store that water in Spinney, and hold back the releases from Wild Horse.

They are proposing an underground pipeline from Wild Horse to Spinney, which has a variety of advantages over an open channel system. Being underground, it wouldn’t freeze and it wouldn’t suffer evaporative loss. Rather than have the water always going into Spinney (up to their storage limits) when there is runoff, the water could be moved on as-needed basis throughout the year.

So far, the project seems to have only benefits:
– The land it is being built on is private
– The reservoir is off-stream, so it’s not impacting any waterbody
– No new water rights are involved — nothing is getting de-watered elsewhere
– The design is smart, prioritizing depth and smaller surface area
– It’ll allow Aurora to better leverage the runoff from different watersheds
– It increases their overall storage capacity by about 60%
– They will be in talks with CPW about making it available to recreation.

A few longer term questions come to mind:
– Will the Spinney inflow be of drastically different temperature to affect that fishery?
– Any chance for some invasive species to take hold in the new reservoir and travel downstream to others?

Aurora Water is now going through the process of securing all permits, doing environmental assessments, etc. If the permitting goes through as hoped for, they plan on start construction in about three years.

In this article, I have not touched on Aurora’s conservation work — the Praire Waters plant which recycles used water. How native water can be used only once before having to pass onto the next water right owner, but how transbasin water can be used over and over.

For more information about this project, check Aurora Water’s Wild Horse reservoir fact sheet.