Hydropeaking Operations of Two Run-of-River Mega-Dams Alter Downstream Hydrology of the Largest Amazon Tributary

Rafael M Almeida,Nathan Barros,Alexander S Flecker,Alexander J Reisinger,Fábio Roland,Carolina R C Doria,Stephen K Hamilton,Ayan S Fleischmann,Emma J Rosi

doi:10.3389/fenvs.2020.00120

Abstract

© Copyright © 2020 Almeida, Hamilton, Rosi, Barros, Doria, Flecker, Fleischmann, Reisinger and Roland. Large storage dams have widely documented impacts on downstream aquatic environments, but hydroelectric dams with little or no capacity for storage of water inflows (i.e., run-of-river) have received less attention. Two of the world’s largest run-of-river hydropower dams (Jirau and Santo Antonio, Brazil) are located on the Madeira River, the largest tributary to the Amazon River. Here we examine whether the Madeira dams have affected downstream seasonal flood pulses and short-term (daily and sub-daily) flow dynamics. We show that the combined effects of these dams on seasonal flood pulses were modest. However, dam operations significantly increased day-to-day and sub-daily flow variability. The increase in short-term flow variability is largely explained by rapid, short-term variations in river flow caused by fluctuations in energy demand (hydropeaking). Both the magnitude of hydropeaking and the mean absolute day-to-day change in discharge downstream of the dams doubled after dam closure. In addition, the median hourly rate of water level change downstream of the dams was three times higher than upstream. Our findings highlight that even run-of-river dams on very large rivers such as the Madeira—whose average discharge at the dam site is larger than that of the Mississippi River at its mouth—can alter downstream hydrology through hydropeaking. Although little studied in tropical floodplain rivers, hydropeaking by large run-of-river dams may be detrimental to downstream aquatic organisms and human populations that utilize the river for navigation and fisheries.

Highlights

Dams affect downstream ecosystems and their biodiversity through alteration of the frequency, magnitude, duration, timing, and rate of change of natural flow regimes (Richter et al, 1996; Poff et al, 1997; Nilsson and Berggren, 2000)
The markedly unimodal nature of the Madeira River’s seasonal flood pulse was preserved downstream of the Jirau and Santo Antônio dams after their construction (Figures 3A,C), which becomes especially clear when comparing pre- and post-dam monthly Pardé coefficients (Figure 4C)
Our results suggest that the two large run-of-river dams recently built on the Madeira River have not altered downstream seasonal flood pulses, which was anticipated in pre-dam environmental impact studies

Summary

Introduction

Dams affect downstream ecosystems and their biodiversity through alteration of the frequency, magnitude, duration, timing, and rate of change of natural flow regimes (Richter et al, 1996; Poff et al, 1997; Nilsson and Berggren, 2000). Most of the existing knowledge on the downstream impacts of dams comes from storage dams with relatively large reservoirs, high flow regulation, and long water residence times, which cause significant disruption of downstream flow regimes (Lehner et al, 2011). Most studies on the downstream impacts of run-of-river dams have examined relatively small dams located in North America and Europe (Anderson et al, 2015; Bejarano et al, 2018). It is not known how modern, large run-ofriver dams such as those newly constructed, under construction, and planned for the Amazon basin (Anderson et al, 2018; Almeida et al, 2019b) may affect downstream flow regimes. Understanding the hydrological effects of contemporary Amazon dam operations is especially important considering that past dam construction has caused substantial hydrological alterations in some Amazonian rivers (Timpe and Kaplan, 2017)

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Results

Conclusion