Zero-sum game (noun): A situation in which a gain by one person or side must be matched by a loss by another person or side. (From here).
The narrative of dryland water use as conflict, even warfare, is firmly entrenched in the Western cultural tradition. “Water wars” is a memorable alliterative phrase that paints the division of our water resources as a zero-sum conflict: one side wins, the other side loses. The water cannot be in two places at once.
The problem with this narrative is that it is mostly untrue. Water is indeed in many places at once and provides multiple benefits. Surface water in Colorado is used and returned to the river three to seven times before leaving the state’s boundaries- and possibly even more (read more on page 20 of this link). Upstream irrigators divert more water than their crops can absorb, and the remainder returns to the river or shallow aquifer as “return flows.” The same happens with municipalities- their treated wastewater is sent downstream for use by others. In these cases, a water diversion is not necessarily a zero-sum game; while consumption by plants, animals and humans dries up the stream a little, the return flows reduce the impact on downstream users.
Not so with transbasin diversions. If you’ve read my previous entries in this series, you’ll know that these critical projects remove water from an entire river system and deposit it in another. Once that water is gone, it’s gone. In other words, when a new diversion is built across a drainage divide, one basin’s loss is the other basin’s gain. Transbasin diversions are truly a zero-sum game.
This post will examine impacts of transbasin diversions on fish and wildlife habitat and environmental quality in the basin of origin- impacts that are entirely different from those of diversions within a single watershed.
Stream Morphology and Aquatic Habitat
Aquatic habitat- that is, habitat within a stream rather than on its banks- depends on a great number of conditions. Several of the critical factors are dependent on the amount of water physically in the stream channel. Most notably, water levels affect habitat connectivity and water temperature. I’ll allow this video from Colorado Trout Unlimited, a conservation and sportsmen’s organization, to do some of the explaining for me.
Lowering water levels increases the width-to-depth ratio of a stream, exposing more rocks to sunlight and more of the stream bottom to warming by solar radiation. Colorado’s native trout are extremely heat-intolerant, requiring home waters in the range of 10 to 20 degrees Celsius (50 to 68 degrees Fahrenheit). Decreasing depth also negatively impacts trout’s ability to move up and downstream through shallow riffles like those at the beginning of the above video. Studies of the Dolores River in southwestern Colorado have found that serious declines in native fish populations are at least in part due to insufficient flows following transbasin diversions and the resulting effects on temperature and connectivity.
Zero-return diversions also have a profound effect on the timing of water flows through streams. Normally, a stream hydrograph (plotting flow rates against time over the course of a year) might look something like the graph on the left:
Now look at a hydrograph on the right, showing the same river downstream, below the massive Adams Tunnel storage and diversion system. The flow rate values on the y-axis are higher overall because this is below the confluence with the Fraser River, Willow Creek and other important tributaries. But the hydrograph is much broader– it leaves “base flow” earlier, ascends more gradually, and descends more gradually back to base flow. It also includes more irregular peaks.
The importance of these two graphs is that the first has more significant spring flushing flows. These are the steep flows in late spring and early summer, associated with snowmelt, that are most pronounced on unimpeded, undiverted rivers. Flushing flows provide a variety of functions on alpine streams, especially sediment removal, contaminant dilution and removal, and inundation of floodplains and streambanks, which benefits streamside vegetation (which provides fodder for wildlife and shades the stream channel, reducing temperature). The potential effect of permanently removing water from a stream during high-water periods in the spring and early summer is represented abstractly in the following graph:
You can see why conservation groups and sportsmen’s organizations have asked for guarantees of strict environmental protection from the two currently proposed transbasin diversion expansions, through the Moffat Tunnel and Adams Tunnel, in the Colorado River headwaters. Early transbasin diversions, including the construction of those very tunnels, did not have to reckon with an aware, motivated environmental and recreation caucus on the Western Slope. It speaks to the progress of environmental awareness over the last several decades that both water providers have agreed to habitat mitigation measures, with Denver Water going so far as to say “the west slope will be better off with [the Moffat Firming Project] than without it.” The agreed mitigation elements include 1000 acre-feet of water annually for the purposes of sustaining the Fraser River in dry years and spending $25 million on water quality and habitat improvements on the west slope. This has not been enough for Trout Unlimited, which said this summer that it would only support the project if Denver Water goes further in protecting the river by ceasing diversions entirely during very high water temperatures and spring runoff.
Luckily, both sides are committed to finding a mutually acceptable agreement rather than pursuing a unilateral “Water Wars” solution. Northern Water, for their part, reached an agreement with Trout Unlimited and west slope interests to construct a bypass around or through Windy Gap Reservoir to maintain water temperatures and flushing flows, to provide water to Grand County for environmental purposes, and to spend money on habitat restoration. Apparently even zero-sum games can be amicable.
The Other Side of the Equation
If this is all a zero-sum game, who benefits? Obviously, the rivers of the receiving basin should capture 100% of the water diverted underground. Unsurprisingly, the documentation of benefits to east slope rivers themselves (as opposed to east slope communities, farmers and ranchers) is pretty sparse; people seem to be most concerned about fish habitat when it is under threat, not when it is being augmented. I’ll cover economic and social impacts on both receiving and originating basins in future posts, but I want to briefly address the benefits to fish, wildlife, and stream health in receiving basins.
There’s no doubt that maintaining or increasing instream flow in any river is beneficial to aquatic and riparian life, for the same reasons that depleting rivers is harmful: improved depth is good for water temperature and habitat connectivity, higher flows recharge groundwater and inundate floodplain vegetation, and higher flow rates dilute contaminants and transport sediment and chemicals downstream (although, if the increased flow comes out of a regulated diversion, it is unlikely to take the form of strong spring flushing flows). That being said, it seems unlikely that the benefits to a “gaining stream” are anything close to the costs to a “losing stream,” unless the gaining stream was itself deficient in water before receiving the transbasin diversion. Streams don’t necessarily benefit from having consistently more water in the same way that they suffer from having consistently less. In an interbasin transfer of water, the water itself may be zero-sum, but the impacts on aquatic and riparian habitat are almost certainly not.
This is the third post in a six-part series fulfilling requirements for ENVS 5000, Policy, Science and the Environment, at the University of Colorado Boulder.