Tributaries may reset the legacy of dams in large rivers

Tributaries, river management and damming. In this post, Associate Editor Tadeu Siqueira comments the recent article by John Sabo and colleagues, Pulsed flows, tributary inputs, and food-web structure in a highly regulated river.

The damming of large rivers has been common practice, probably since Thomas Edison built one of the first hydroelectric stations in 1882. Most important rivers in economically developed regions have already been dammed, and many others, like the Congo and the Amazon, have been  undertaking this process in recent years. There is also little doubt that dams are, to some extent, beneficial to people. If well-planned, they provide electricity and water to houses, farms and industry. Indeed, dams have become part of our landscapes and even culture – picture them in famous film scenes from Superman, X-Men and GoldenEye, to mention just a few.

The Glenn Canyon dam, in the Colorado River, US. (Photo: US Geological Service)

However, some may also argue that dams have negative effects on both the natural functioning of rivers and the lives of local human populations. Dams fragment rivers, altering their hydrology, biology and energy flow. For example, there is evidence that the fertility of wetlands located downstream reservoirs is severely reduced because dams retain sediments and nutrients upstream. Some studies also show that changes in the frequency of discharges in regulated rivers negatively affect fish recruitment by modifying the environmental signals that trigger gonad maturation, migration and spawning. This crossroad between the benefits and impacts of river damming has puzzled ecologists and conservationists for decades. How great would it be to make rivers downstream from dams more natural, without spending too much money and with low intervention?

Some ecologists suggest that tributaries flowing towards large rivers may play a fundamental role in mitigating the downstream impacts caused by dams. The idea is that tributaries that are not fragmented and regulated by dams may help create physical discontinuities in terms of discharge, due to their seasonal flows. This then augments the opportunities for movement of various aquatic organisms, such as insects, crustaceans and even big fish species, which are severely threatened by dams worldwide. Also, because tributaries usually run within more pristine and forested watersheds, they may deliver sediments and coarse organic matter, like leaves and debris, to the main channel. These are some of the hypotheses Sabo and colleagues investigated in a new article in Journal of Applied Ecology. They studied the highly regulated Colorado River in the Grand Canyon (USA) and seven major tributaries that collectively increase the drainage area upstream of Glen Canyon dam by 34%. Sabo and colleagues were mainly interested in understanding the contribution of these tributaries to the Colorado River in terms of organic matter, basal resources to food webs and the structure of food webs. To do so, the scientists combined information from different types of organisms (e.g., algae, amphipods, insects, and fish), modern laboratory techniques, like analysis of stable isotopes, and robust statistical modelling. They gathered all the information they needed following a sampling design that included a longitudinal sampling scheme; from the dam to the last tributary, located 388 km downstream. They repeated this design in two distinct seasons, pre-monsoon and post-monsoon. With all that, they could test if food web structure changed according to the distance from the dam, and if that varied between seasons.

Members of the USGS monitor the Colorado River downstream of Glen Canyon Dam during the high-flow experiment. (Photo: US Geological Service)

Conducting observations at the spatial scale of Sabo and colleagues’ study is not an easy task – almost 400 km reach of the Colorado River, including seven tributaries and sampling a huge amount of material in two seasons. It would be virtually impossible to get independently replicate of this kind of information from different rivers. Under these circumstances, when replication at the appropriate spatial is not feasible, one has to put together numerous lines of evidence to support an idea. That is certainly what the authors did. First, they found that basal resources for food webs were predominantly autochthonous algae close to Glen Canyon dam, but reverted to allochthonous (terrestrial organic matter coming from tributaries) within ~ 168 km of the dam. Second, they found that primary consumers relied on terrestrial food sources right after the first tributary, but only after post-monsoon floods. Third, fish (secondary consumers) also changed their food items along the longitudinal gradient, with a peak in allochthony between the middle of gradient and the last tributary. Finally, they found that food chain length also varied from the dam to the last tributary; during the pre-monsoon season the complexity of food webs increased (e.g. more trophic levels) towards downstream of the dam. After the monsoon, foods webs were, in general, more complex, especially after the mid-Grand Canyon. Therefore, although Sabo’s study was based on only one system, the various lines of evidence collectively support the idea that tributaries increase the relative importance of terrestrial materials to the functioning of regulated large rivers, thus ‘resetting’ the diversity and food web structure downstream of a dam, especially during monsoon seasons.

The findings made by Sabo and colleagues have clear implications for the management of river networks worldwide. The most obvious is that tributaries with seasonal or variable flood dynamics should provide conservation value for large rivers with hydropower cascades. As historically large rivers are the main targets for damming, protecting their tributaries seems to be a sound management plan. As explained by Sabo and colleagues, ‘a modest set of reasonably sized tributaries could be used to provide seasonal variation and enhance production of non-migratory or locally migrating fishes vital for food security in rivers where inland fisheries prevail’. The problem is that dam construction has already started to target not only large rivers, but also its tributaries. Worryingly, in some regions, large unregulated tributaries might not even exist. For example, Macedo and colleagues (2013) estimated the existence of 10,000 impoundments in tributaries of various sizes in the upper Xingu basin, Brazil. This is unlikely to be an exception, urging scientists, managers and decision makers to take the next step towards more comprehensive management, not just of large rivers, but also their entire river networks.

Read the full article, Pulsed flows, tributary inputs, and food-web structure in a highly regulated river in Journal of Applied Ecology.




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