The roaring Fork is Roaring
June 23, 2017
The Roaring Fork River has certainly been living up to its name this past week, reaching peak flow thanks to a combination of steady warm temperatures, melting snow in the high country, and the suspension of the Twin Lakes diversion near the river’s headwaters (more on that in a minute). As water recreationalists rejoice during this period of high flow, water managers and conservationists have been keeping a close eye on how the annual spring runoff progresses. This post will explore how the upper Roaring Fork watershed, particularly the North Star Nature Preserve east of Aspen, is at the core of a delicate interplay between water demand from the Front Range and river ecosystem health in the Aspen area.
|Figure 1. Aerial view of the flooded North Star Nature Preserve in June 2015, creating what is known as “Lake North Star.” Photo courtesy of Greg Poschman.|
One of the most noticeable impacts of this high, post-diversion streamflow is the flooding of North Star Nature Preserve east of Aspen – a mellow stretch of the river where it meanders through a natural floodplain (Fig. 1). As discussed in an ACES blog post last June, we have previously observed a “sponge effect” in the North Star where the river overflows its main channel and spreads out into its floodplain, creating what the locals refer to as “Lake North Star.” So why does this matter? Now that we know that this “sponge effect” exists (this year’s streamflow data will be discussed later in this post), is repeated spring after spring, and is directly linked to the Twin Lakes diversion, we can begin to consider this phenomenon in the broader context of water management and river ecology in Colorado. First off, why are high spring flows important for the river ecology? Those high flows cause the river to overflow its banks, bringing water to different parts of the preserve which in turn expands the riparian areas that are excellent habitat for a diverse array of flora and fauna. Furthermore, certain plant species rely on the periodic flooding of wetlands to survive, as this process helps with seed dispersal and nutrient flow. This annual spring flooding also helps to recharge the groundwater table in the area, which is critical for the drier summer months that follow. The entire river ecosystem depends on those high spring flows, flows that are now directly linked to the status of the Twin Lakes diversion.
The Roaring Fork typically peaks in early to mid-June, but some might be surprised to learn that the dominant control on the timing and magnitude of this annual peak is not simply snowpack and temperature but rather the Twin Lakes diversion. We have a unique situation here in Colorado – 80% of the water is found on the Western Slope but 80% of the people live along the Front Range (CO Water Conservation Board). To supply enough water to the millions of people living on the Front Range, 24 tunnels have been drilled through the Continental Divide to transport water from west to east. The Twin Lakes diversion – the 5th largest in the state – takes water from Grizzly Reservoir in the upper portion of the Roaring Fork watershed through a 4-mile-long tunnel where it is released into the Twin Lakes Reservoir and eventually distributed to the cities of Aurora, Pueblo, and Colorado Springs. The Twin Lakes Reservoir & Canal Company has a right to 46,000 acre-feet of water every year, equivalent to almost 15 billion gallons or 22,710 Olympic-sized swimming pools! This typically translates to a diversion of ~600 cubic feet per second (cfs) for a 2 to 3 week-long period in the late spring. More information about the Twin Lakes diversion can be found here.
Figure 2. Twin Lakes Tunnel discharge this June. Note the shut off of the diversion on June 14th, dropping from over 600 cfs to 0 cfs in less than a day.
Figure 3. Streamflow in the Roaring Fork River pre- and post-diversion shut off on June 14th. Note the delayed increase in downstream discharge, peaking on June 19th.
In early June, the Roaring Fork was running at ~400 cfs through Aspen while the Twin Lakes tunnel had ~600 cfs going through it while it was open (Figs. 2 & 3). Thus, when the diversion was shut off on June 14th, the discharge coming down the river more than doubled! The post-diversion discharge measurements represent close to the “native” flow of the Roaring Fork as it enters Aspen. The ACES stream gauge in Aspen recorded a peak discharge of 1321 cfs at 1 am on Monday, June 19th with a subsequent daily decrease in peak daily discharge every day since (Fig. 3). The current daily fluctuation is in the neighborhood of 250 cfs with a peak discharge between 4 and 7 am that reflects the delay from snow melting in the upper watershed the prior afternoon and percolating into small creeks before eventually entering the Roaring Fork River and flowing downstream to Aspen. Figure 4 shows the dramatic rise in the river over the course of just a few days.
|Figure 5. Satellite view of North Star Nature Preserve showing the old channels and abandoned meanders that serve as the “sponge” for the Roaring Fork River.|
Now back to the flooding of North Star. As we have done in for past years, we can analyze the discharge measurements from river gauges upstream and downstream of the preserve to get a more quantitative handle on the effect of flooding that natural floodplain (Fig. 5). In the low-flow conditions (<500 cfs) that exist prior to the diversion shut off, the downstream gauge (Stillwater) typically records higher flows than the upstream gauge (Difficult). This is because the Roaring Fork is a gaining stream, meaning that it continues to increase in flow as it moves downstream due to numerous inflows from smaller tributary streams. However, in the high-flow conditions (>500 cfs) that exist after the diversion is shut off, a curious thing happens – the upstream Difficult gauge begins reporting higher flows than the Stillwater gauge downstream (Fig. 6). This seems counterintuitive for a gaining stream like the Roaring Fork, but the explanation lies in the overflow in the preserve that creates “Lake North Star.” The abandoned meanders, channels, and pools in that area soak up much of the excess flow like a sponge and hold it in the preserve, leading to increased water loss to evaporation as well.
|Figure 6. Comparison of discharge measurements for river gauges on either side of North Star. Stillwater gauge = downstream; Difficult gauge = upstream. The yellow line marks when the diversion was shut off, allowing for higher flows in the Roaring Fork. Note that the discharge on the upstream gauge briefly exceeds that on the downstream gauge at various times.|
In a typical year, 38% of the total volume of the flow in upper Roaring Fork watershed is diverted to the Front Range (Roaring Fork Conservancy). Water right laws in Colorado assure that the full allotment of diverted water is available each year regardless of the Roaring Fork’s discharge in a given spring. Following low-snow winters. the diversion could result in the river never having high enough flows to naturally flood North Star and the ecological benefits that accompany that process. The challenge will be to balance the regional water needs with the local ecological needs, which will require the different stakeholders – water managers, federal/county land agencies, conservationists, local residents, etc. – to work together each spring to make a plan for this critical time period for the river. For instance, the diversion could be shut off for a week every June to allow the river to receive its natural peak flows and flood North Star. The diversion could then resume if necessary to deliver additional water to people living in the Front Range. A little flexibility all around from the stakeholders involved would go a long way to ensuring a long and prosperous future for the Roaring Fork River and those who rely on it.