Essentially the authors are starting from the direct effects of an impoundment and extrapolating out what the effects would be on downstream channel morphology and flow, and subsequently downstream biota. In this case, this hierarchy has been laid out in general terms in a previous paper (Jorde et al. 2008). Basically first order stuff is what causes the second order stuff, and the third order stuff is caused by the first and second order stuff, and so on, and so on.
In this study, the first order stuff is the hydrology, the second order stuff is "channel hydraulics" and bed mobility, and the third order stuff is the recruitment of cottonwood trees. Channel hydraulics seems to be referring to the depth and discharge of the streams being investigated. The entire study is of the Kootenai River (a trib of the Columbia River), which includes two dams: The Corra Linn Dam (built in the 1930s on a naturally occurring lake) and the Libby Dam (finished in 1974). Both dams are ostensibly dual purpose: hydropower and flood control. In addition, there were lots of levees constructed along the reach.
Basically the authors compared 3 time periods: Before all the dams, before the Libby Dam, and After the Libby Dam. Apparently the Libby Dam is the subject of some controversy, which is why it is getting such special attention.
Let's start with the first order effects. They are actually pretty dramatic, but then again, this is why the dams were built. These are the effects we expected when humans built the dam.
This if Figure 6 from the Burke et al. paper. What we're looking at is the difference between the discharge pre-Libby and post-Libby (the other dam doesn't seem to have as much of an effect as the Libby dam). See the big negative values for the spring? That's the spring snowmelt being intercepted by the dam. The big positive values? That's when they are releasing that snowmelt (in the winter!). Considering the importance of hydrology to all kinds of ecosystem properties in streams, that's a big impact.
Now, what does that mean for the second-order effects? Well, I'm not going to sample the figures of this (which are huge and beautiful but take some time to process), but what changes a lot is the water velocity and the wetted width. Instead of periods of extremely high velocity followed by periods of extremely low velocity, you get prolonged periods of more moderate velocity. This is especially true in the first 80-100 km downstream of the dam, where the stream is still relatively steep.
As for third order effects: Well, the authors use cottonwood recruitment as their parameter, which is an interesting choice. I have never really thought of cottonwood trees as a particularly good indicator species, but maybe they used it because the data is available. And boy, the data seems to be available. Basically, for a cottonwood seedling to become established, there have to be:
- High flows to clear out some barren ground so the seeds have somewhere to establish.
- Slowly lowering water levels so enough soil moisture is retained to promote growth in the seedling.
- Flows that rise only to within a particular band above baseflow
- A minimum number of flood events to prevent mortality through the year.
Sound complicated? Actually, not really does it? Regardless, all that data is either known or can be calculated. In fact, it was calculated as part of the second order effects. So all the authors had to do was model this to create estimates of the 'potential' for cottonwood recruitment.
What they found is post-Libby, there isn't much recruitment occurring at all, and even pre-Libby but post-Corra Linn, recruitment was probably pretty rare. By contrast, the historic conditions probably had a very high potential for recruitment. Oddly enough, this was particularly true for dry years. Mechanistically, the model is being a little confused at this, because you need wet years to scour out open patches. Still, this model is basically suggesting recruitment isn't happening much at all post-Libby. A fair amount of empirical work seems to back up this result.
There's a lot more detail here that I won't go into. The whole goal of this paper seems to be to point out how to mechanistically link the known, 1st order effects of an impoundment to the downstream biotic responses. I think this is actually pretty successful, although I'd like to see it used on more interesting biotic response variables than just cottonwood recruitment (diversity indices spring to mind as a potential example). One thing that is really needed for such studies, however, is good historical discharge data. The USGS maintains several hundred gaging stations all over the country, but they've been continually forced to shut those down as budgets are cut.
This is an approach that I think is useful for a variety of other research as well (i.e., agricultural activities, etc.) just because it starts at the basic, well-understood mechanistic level and works up.
The authors document the first, second, and third order effects of dam construction on the Kootenai River. They demonstrate a strong effect of two dams on the hydrology, channel morphology and hydraulics, and the recruitment of cottonwood trees in the downstream reaches. This approach could be a useful framework for investigating other effects of impoundments when primary impacts are known.
Jorde, K., Burke, M., Scheidt, N., Wlcker, C., King, S., Borden, C. 2008. Reservoir operations, physical processes, and ecosystem losses. In: Habersack, H., Piegay, H., Rinaldi, M. (Eds.), Gravel-Bed Rivers VI: From Process Understanding to River Restoration. Elsevier, pp. 607-636.
Burke, M., Jorde, K., & Buffington, J. (2009). Application of a hierarchical framework for assessing environmental impacts of dam operation: Changes in streamflow, bed mobility and recruitment of riparian trees in a western North American river Journal of Environmental Management, 90 DOI: 10.1016/j.jenvman.2008.07.022