The Greek philosopher Heraclitus mused more than 2,500 years ago that “no man ever steps in the same river twice.” He had been pondering the supreme significance of change in the universe, but I like the quote simply because it might be the first time anybody had conveyed the fundamental nature of rivers. In a nutshell, rivers change – they vary. This natural variability is the essence of rivers. It affects how rivers work, in turn affecting which organisms live in rivers. However, we have altered this natural variability by building dams on rivers to store and divert water for many purposes, including drinking and irrigation water and electric power. Dam construction may have started as far back as 2650 BC, with Sadd el-Kafara in Egypt, but it did not begin in earnest until the mid-1900s, with Hoover Dam in Nevada/Arizona (read Marc Reisner’s Cadillac Desert for a thorough account of dam building by the Bureau of Reclamation and the Army Corps of Engineers in the American West), and now we have more than 75,000 dams over 2 meters high in the United States alone. Although dams have many benefits, such as deterring floods that devastate cities and droughts that wither crops, floods and droughts themselves are natural features of river flow regimes, and our suppression of them has produced many costs for rivers.
The natural flow regime of rivers is described as comprising five components that are critical to rivers and the organisms that rely on them. These components include the magnitude, frequency, duration, timing, and rate of change of flows, and many plants and animals have evolved adaptations to them. For example, plains cottonwood is flood dependent along rivers in the western USA, where the natural flow regime is dominated by snow melting during the spring months. The early summer peak flow erodes vegetation from a river’s banks, and deposits sediment in other parts of the river channel, creating new ground for trees to occupy. Adult trees release seeds when the peak flow begins to recede, and the seeds land on this new substrate and germinate. As the peak flow continues to recede, the roots of the new seedlings grow deeper, toward the water-saturated soil beneath the surface. Because the flooding occurs with some predictability, older saplings are recruited into adulthood, and new seedlings can establish on new habitat created with the next flood. However, dams have disrupted this predictable flooding, and plains cottonwood has been disappearing along many rivers downstream from dams while many non-native tree species, such as saltcedar, have been replacing them.
We need water from rivers for many purposes, and dams are our means for fulfilling those demands, but dams are harmful to many organisms that need water in rivers as well. This simple realization led us to consider ways to obtain water from rivers while reducing our impact on organisms that depend on rivers. For instance, an early impact of dams was that rivers could sometimes run dry because we demanded too much water (all of it) during droughts. To prevent this drying of rivers, we decided that rivers should always retain some “minimum flow” that we would be obligated to meet, typically because it benefited some valued game fish. However, as we developed a better appreciation for the natural dynamism of river flow regimes, we began to realize that this initial focus on minimum flows was too simplistic to sustain healthy rivers. We understood that water management needed to maintain some semblance of the natural variability of rivers, initially arising as a question of “how much water does a river need?” Yet, the answer was primarily approached from the perspective of rivers as legitimate users of water, and did not explicitly address the predominating human dimension of water demands on rivers. Toward that end, a more comprehensive strategy for managing river flow regimes has finally emerged with the idea of “environmental flows”.
Environmental flows are defined as “the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems”. As this definition establishes, environmental flows do not entail river flow regimes that sustain just organisms living in rivers but also we who rely on water from rivers. This definition also conveys that at the heart of river sustainability is the balancing of a social-ecological system that governs how we and other organisms benefit from rivers. A way that we may achieve this balance is through the re-operation of dams to restore a more natural flow regime to rivers, mitigating some of the negative effects of flow regulation on organisms while also still delivering water that meets most of our needs. An example that shows the promise of dam re-operation is three experimental high-flow releases from Glen Canyon Dam on the Colorado River, which were designed to transport sediment into the Grand Canyon and create habitat for endangered humpback chub while minimizing the impact on our water use. Yet, this balance is not quickly or easily achieved with existing water law in the western USA under “beneficial use” and “prior appropriation” doctrines (read the Colorado Foundation for Water Education’s Citizen’s Guide to Colorado Water Law for more information on water law), but we can attempt to provide tools that will support decisions about how water might be distributed toward attaining this balance.
My research is an attempt to develop tools that will support these decisions. I am trying to show how to restore a more natural flow regime to rivers that will aid native species of concern while also inhibiting non-native species. Moreover, I am interested in how climate change will affect future water supplies and what that will mean for balancing our demands for water from rivers and the needs of organisms living in rivers. Lastly, an important impact of reservoirs on rivers is that they change the water temperature downstream from dams, warming the water during the winter months and/or cooling it during the summer months. Because water temperatures are expected to increase with climate change, the release of cooler water from reservoirs may be beneficial to some cold-adapted fishes, like cutthroat trout. The tools I am developing will hopefully inform water managers on how to re-operate dams to positively affect not just river flow regimes but their thermal regimes as well. I am applying these tools to the Colorado River, its tributaries and their dams, and I hope to demonstrate the feasibility for releasing (or not releasing) water at certain times of the year and at certain places in the basin to provide the greatest benefit to native species of concern while producing the least cost to us in lost water use.
Climate change and population growth in the western USA will present many challenges in the years ahead (read the US Bureau of Reclamation’s Colorado River Basin Water Supply and Demand Study for more information on potential scenarios), but I think that we can decide as a society how to attain a balance between our demands for water from rivers and the needs of plants and animals that rely on a more natural flow regime for their livelihood and continued existence. Many of the needs of humans and other species fortunately are not diametrically opposed and can be met through similar river flow regimes. Water use in the western USA is not a zero-sum game, with humans winning only if other species lose. Life is full of trade-offs, and I hope my research will support decisions that reduce harm to rivers while maintaining a reasonable semblance of our way of life in the western USA. It is impossible to have our cake and eat it too, but if we can think strategically about how water is distributed—when and where—it may be possible to have our cake (sustain healthy rivers) and at least lick the icing off (satisfy most of our water demands).