Any time-series of discharge or flow data can be broken into five functional sub-components: 1) flow magnitude (i.e., the total volume of water transported per unit time); 2) the frequency of a given flow event (e.g., number of overbank floods per year); 3) the duration of a given flow event; 4) the timing of a flow event (i.e., day of the year); and 5) rate of change. And any one of these five sub-components may determine the presence or abundance of a given fish species. This logic underpins all research on 'ecological flows.'

With major climate-driven shifts in regional and global hydrology on the near horizon, the importance of environmental flows research cannot be overstated. It is one of the best tools that we have for predicting how aquatic species may respond to changes in hydrology.

I am using a similar approach in my research, but I am more focused on large-scale patterns in overall aquatic diversity than in the ecology of any one species. For instance, I've shown that fish species richness has a similar, positive association with mean annual discharge in the highly diverse rivers of the Southeast U.S. and the depauperate rivers of the Pacific Northwest.

Several interesting themes emerged from the Pacific Northwest analyses. Predictably, fish species richness was positively correlated with flow magnitude. But the strength of the positive correlation was greatest during the low flow winter months (as shown at left). Richness was also inversely correlated with intra-annual variability for most flow metrics.

Most recently, I've begun a much larger scale study of fish-flow associations in Western Hemisphere rivers with Bianca de Freitas Terra (a past post-doc in my lab, now a researcher at the Federal University of Ceará in northeast Brazil). We're using long-term flow records from 107 rivers distributed throughout North, Central, and South America (study basins are outlined in the map below) to test whether the kinds of flow sub-component patterns that I documented in Pacific Northwest rivers are germane. So far, we have confirmed that some of the previous results are broadly applicable, but we've also documented some key differences between tropical, subtropical, and temperate rivers and we're working to get these results published ASAP.