Making a connection: interaction between large dams

In a case study of dams on the Upper Missouri River, US Geological Survey (USGS) researchers have demonstrated that an upstream dam is still a major control of river dynamics where the backwater effects of a downstream reservoir begin. In light of this finding, the conventional understanding of how a dam can influence a river may have to be adjusted to account for the fact that effects of river dams can interact with one another.

"We have known for a long time that dams have dramatic effects on river form and function," said Jerad Bales, acting USGS Associate Director for Water. "In the past, however, the effects of dams generally have been studied individually, with relatively little attention paid to how the effects could interact along a river corridor."

The downstream effects of river dams have been well documented by previous researchers. In the presence of a dam, it can often take hundreds of kilometres for a river to adjust to its natural state. The upstream impacts of dams have also been widely considered, particularly the sedimentation of reservoirs: the effects of which may extend upstream for many kilometres. However despite such significant advancements in knowledge, these are often considered independently of one another.

According to the authors, the current governing hypothesis is that the effects of dams attenuate in space and time both upstream and downstream of a dam until a new equilibrium is reached in the system. But given the extremely long distances required for attenuation it may frequently be interrupted by other dams. The authors of the study, which focused on the Garrison and Oahe dams and was published in the journal Anthropocene in 2013, hypothesise that where dams occur in a longitudinal sequence, their individual effects interact in unique and complex ways with distinct morphodynamic consequences.

The report states that: "On the Upper Missouri River, the Garrison Dam reduces both the supply and changes the size composition of the sediment delivered to the delta formed by the reservoir behind the Oahe Dam. Conversely, the backwater effects of the Oahe Dam cause deposition in areas that would be erosional due to the upstream Garrison Dam and stratifies the grain size deposition. These effects are further influenced by large changes in water levels and discharge due to seasonal and decadal changes in dam operations."

The study introduces the concept of a distinct morphological sequence which is referred to as an inter-dam sequence. The authors proposed that this inter-dam sequence is simultaneously impacted both in the downstream direction by a dam upstream, and in the upstream direction by a dam downstream. It is also thought to be likely that this inter-dam sequence is prevalent on most large rivers in the US and potentially common across the world.

"In addition to documenting dramatic changes to a section of the Missouri River during the 2011 floods," Bales continued, "the unique contribution of this important study is the development of a conceptual model that establishes a framework for future studies of the many rivers affected by dams in series."

Working with historical aerial photography, stream gauge data, and cross-sectional surveys in a careful analysis of the Garrison and Oahe Dams, the USGS researchers developed a conceptual model of how interacting dams might affect a river's physical characteristics (geomorphology). This model applies to dams on large rivers and divides the river into various zones of predictable behaviour (see figure 1).

By using the examples of the Garrison and Oahe Dams, the authors were able to demonstrate that the effects of an upstream dam maintains significant geomorphic control over river morphology as the backwater effects of the downstream reservoir begin to occur. The report states that five unique geomorphic gradational reaches were identified for the Garrison Reach, two of which are controlled solely by the upstream dam and three of which are controlled by the dam interaction.

The researchers also conducted a geographic analysis of dams along 66 major rivers (as listed in a standard professional reference) in the contiguous US to determine how often dams occur in a series. Of the 66 major rivers analysed, 404 dams were located on the main stem of 56 of the rivers. Fifty of these rivers had more than one dam on the river creating a total of 373 possible inter-dam sequences. The average distance between these dams is 99km.Thirty-two percent of the inter-dam sequences had lengths of 25km or less; 41% were less than 100km; and 26% of the dams were within 1000km of one another. Only one inter-dam sequence was identified to be longer than the 1000km. These results suggest that there are numerous large dams occurring in sequence on rivers in the US and that the distance between the majority of these dams is much closer than the hundreds of kilometres that may be required for a downstream reach to recover from an upstream dam.

With the study results indicating that more than 80% of large rivers may have interactions between their dams, the USGS investigators suggest that dam interaction is prevalent and should be the focus of additional research.

Reference

Large dams and alluvial rivers in the Anthropocene: The impacts of the Garrison and Oahe Dams on the Upper Missouri River by Katherine J. Skalak, Adam J. Benthem, Edward R. Schenk, Cliff R. Hupp, Joel M. Galloway, Rochelle A. Nustad and Gregg J. Wiche. Published in Anthropocene 2 (2013) pp51-64

The full report can be viewed at science direct.