A Watershed is More Than the Sum of its Reaches: Understanding Post-Fire Resilience at Different Scales

Jan 9, 2024

Contributed by Shayla Triantafillou, MS Student, Colorado State University

A wide, well-connected floodplain in a resilient watershed.

Wildfires are one of the biggest threats to water in Colorado. The ten largest fires in Colorado history have happened since 2002 [1]. Fires change the hillslope and river corridor properties like vegetative ground cover and soil structure, which in turn affect processes including vapotranspiration, runoff generation, and erosion [2, 3]. Stand-killing wildfires therefore cause larger floods that carry more sediment. As wildfires increase in frequency and severity in a changing climate, there is a growing need to understand the cascade of effects that follow fire. This has been especially pressing in Colorado after several major fires burned in 2020. The Cameron Peak fire of 2020 burned over 1600 stream kilometers (1000 miles) [6] and has been a focus of river researchers, practitioners, and management agencies in the Front Range. The primary concerns are the increased water and sediment inputs from burned watersheds, which results in reduced fish habitat, poor water quality, and in the most extreme cases, debris flows impacting human lives.

In the three years since the fire, summer convective summer storms over the burn scar have impacted neighboring watersheds differently, indicating that watersheds exhibit differing responses and therefore differing resilience post-fire. Some watersheds have seen few changes, while others, such as Black Hollow, had debris flows that greatly altered the forms and processes within the catchment. The differences between watersheds are likely due to a combination of characteristics on the scale of the entire watershed and characteristics on the reach scale, where a reach is a relatively short stream length with consistent valley geometry.

A confined valley with minimal floodplain that incised at a watershed with low resilience.

My research goal is to describe the geomorphic characteristics that promote resilience in these burned watersheds at the watershed and reach scales. Before analyzing data, we assigned a resilience ranking to each watershed based on the sediment deposition at the outlet, where less sediment indicated greater resilience. This sediment deposition varied from excessive sediment that created a large fan to no evidence of sediment deposition at the outlet. Within the seven study watersheds in the Cameron Peak Fire burn zone, there were no associations between catchment-scale characteristics (e.g., slope, concavity, burn severity, watershed area, stream length) and resilience. Preliminary results indicate an association between reach-scale river corridor geometry and watershed resilience ranking. Resilient watersheds tend to have more reaches with higher floodplain width relative to active channel width. This aligns with what we already know about wide floodplains promoting attenuation of high flows, and therefore improving resilience.

This trend holds true in all but one watershed. In that case, much of the watershed had low relative floodplain widths, and we started to see evidence of flooding as scour/deposition patterns, much like in lower resilience watersheds. About two-thirds of the way down the watershed, however, the valley widened. Here there were three wide, wet reaches with well-connected floodplains. Below these, we did not see evidence of flooding or of major sediment export from this watershed.

These findings are promising because they show that a watershed’s resilience can be more than the sum of its reaches. A small number of reaches with a capacity for attenuation can have an outsized impact on the resilience of an entire watershed. We can’t change catchment-scale characteristics or even valley geometry at the reach scale, but as post-fire projects in the Front Range have done, we can make sure that the reaches we do have with favorable geometry for attenuation are well-connected and functioning in a way that promotes resilience.

[1] https://csfs.colostate.edu/colorados-forests-changing-climate/

[2] https://pubs.usgs.gov/publication/70023772

[3] https://onlinelibrary.wiley.com/doi/10.1002/eco.2170

[4] https://www.sciencedirect.com/science/article/pii/S0022169400001943

[5] https://onlinelibrary.wiley.com/doi/10.1002/hyp.383

[6] https://www.poudrewatershed.org/cameronpeakfire#:~:text=CAMERON%20PEAK%20FIRE%20IMPACTS%3A,of%20designated%20Wilderness%20Areas%20burned


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