Impact of Dam Clusters on Natural Flow Characteristics of Rivers

A new publication by 2020-21 Global Change Fellow Hemant Kumar and SE CASC Faculty Affiliate Sankar Arumugam was published in the January 2022 edition of the journal, Water Resources Research. This publication, Dynamic Flow Alteration Index for Complex River Networks With Cascading Reservoir Systems summarizes collaborative research by Climate, Hydrology and Water Resources Modeling and Synthesis Group at NCSU and the Risky Waters Lab at CUNY. The collaboration was facilitated by John Wesley Powell Center. The following summary was written by Hemant Kumar.

The natural flow regime of a river refers to its free-flowing state. This natural regime can be altered to meet various human needs such as municipal and agricultural water supply, hydropower generation, flood control, and recreation. But changes in the flow regime can degrade the river ecosystem health, impacting naturally occurring species’ productivity and survival. Most of the major rivers within the continental United States have been artificially altered by thousands of dams. The impacts of such dams have been studied by researchers in the past decades and multiple indices have been proposed to quantify such alteration. This information on various aspects of alteration (also known as regulation) is critical to understand the impact of management strategies on reservoir operations.

Figure 2. Conceptual network with dams and streamflow gages to explain the dynamic degree of regulation methodology. The main stem of the network (thick black line) starts with negligible regulation at gage A followed by introduction of localized regulation at dam 1. The network regulation at gages B and C would differ as a free-flowing tributary, which joins the main stem, will reduce the existing regulation. The network regulation will keep changing as the main stem flows through dam 2 and is joined by a free-flowing tributary and a regulated (dammed) tributary. The network regulation at gage E (no adjacent dam) would be different from that at D and F because two tributaries join the main stem between gages D and E and another tributary joins between gages E and F.

The existing approach uses an index known as residence time (in units of years) to quantify the impact of a single dam by taking the ratio of reservoir capacity and mean annual flow. This approach ignores the cascading effect of dam clusters across the river network. For example, the individual dams of a cluster may each have low regulation value, but the overall regulation of the network compounds as we move downstream due to reservoir operations. Furthermore, this and other existing approaches provide a static regulation value and cannot capture the temporal changes in the regulation. Such changes may happen due to changes in withdrawal demands and climate change. To address these issues, we propose a new Dynamic Flow Alteration Index (DFAI) to compute the cumulative degree of regulation due to dam clusters for complex river networks.

We examine the highly regulated Colorado River Basin to understand how regulation changes from free-flowing upstream reaches to heavily dammed downstream reaches. The Colorado River is the lifeline of nearly 30 million people, 20 Tribal Nations, and numerous species of flora and fauna. Approximately 45% of the river’s water is diverted out of the basin to meet the municipal needs of many cities such as Denver, Phoenix, Los Angeles, and Las Vegas. We use DFAI to estimate the local regulation due to individual dams and cumulative regulation for cluster of dams. The maximum network regulation is 2.52 years at Glen Canyon reservoir. Interestingly, operations of Hoover Dam reduce the cumulative regulation by bringing the altered flow in synchronization with natural regime due to downstream flow requirements (see this related study on how dams alter the river flow at annual and multi-annual scales). The proposed index also captures the impact of San Juan River Basin Recovery Implementation Program, which successfully reduced the network regulation drops by 1.5 years at Navajo Dam.

The proposed time-varying regulation metric captures the human influence due to reservoir operations and water management. DFAI considers the impact of dam operations on flow characteristics such as shifting of peak flow month and dampening of peak flows. This approach will contribute to a more objective evaluation of operating rules for reservoir systems. Our findings from this work emphasize the need to develop naturalized flows for major river basins in order to quantify the flow alteration under continually changing climate and human influences.

Hwang, J., Kumar, H., Ruhi, A., Sankarasubramanian, A., & Devineni, N. (2021). Quantifying dam-induced fluctuations in streamflow frequencies across the Colorado River Basin. Water Resources Research, 57, e2021WR029753.