by Brian Murphy, MS, EI; Engineering & Hydrosystems, Inc.
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Introduction
The mountains and forests of the Front Range between Denver and Colorado Springs are critical for supplying water to communities and cities from the upper South Platte basin. In June of 2002, the Hayman Fire burned large amounts of the basin, severely impacting water quality and riparian ecosystems. Trail Creek, a tributary to the South Platte River, was impacted by the Hayman Fire with a concomitant increase in the sediment yield from the catchment.
This article examines the physical impacts to the Trail Creek watershed including riparian and aquatic ecosystems. A review of forest hydrology, erosion and sedimentation impacts, and recovery process is presented.
Watershed Description
The Trail Creek basin (Figure 1) is approximately 11,000 acres of forested land. The hillslopes vary and can be very steep (up to 25%). The watershed experiences significant grussification (i.e., the granite bedrock is chemically weathered near the surface). Multiple roads run through the watershed, but it is not altered in any other notable fashion.
Willows are abundant in riparian areas. The dominant willow species is mountain willow (Salix monticola), with lesser amounts of plane-leaf willow (Salix planifolia), coyote willow (Salix exigua), and a pruinosa twig willow (Salix drummondiana). Western birch (Betula fontinalis) and thin-leaf alder (Alnus incana ssp. tenuifolia) have also been observed along the creek. Rhizomatous sedges, namely Nebraska sedge (Carex nebrascensis) and beaked sedge (Carex utriculata), are prevalent along the creek in the upper portion. These sedges play an important role in holding the creek banks in place due to their dense and strong rhizomes.
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| Figure 2. Two examples of the direct impacts of high severity wildfire increasing runoff potential; (a above) bank erosion, and (b at right) rilling |
Forest Hydrologic Processes
Soils, vegetation, and litter are critical to the functioning of hydrologic processes. Forested watersheds with good hydrologic conditions (greater than 75 percent of the ground covered with vegetation and litter) sustain stream baseflow conditions and produce little sediment (Robichaud et al. 2003). The Hayman Fire destroyed the accumulated forest floor layer and vegetation and greatly altered infiltration rates by exposing soils to raindrop impact and creating water repellent conditions (Figure 2).
Rills and gullies formed in most depressions and experienced significant downcutting following the fire. Some eroded down to competent bedrock. The pre-fire curve number (CN) was 50 to 60. The post-fire CN was 83. These impacts may last weeks or decades, depending on the fire severity and intensity, remedial measures, and the rate of vegetative recovery.
Over two-thirds of the Trail Creek watershed was classified as either low or moderate burn severity — 38.6% low, 36.5% moderate, respectively — with only ten percent classified as high burn severity. Typically, low and moderate burn severity areas produce less sediment because the vegetation that stops the hillslope sediment from transport is still partially in place.
Beaver dams were prevalent in the Trail Creek basin. The beaver ponds not only altered the hydrology of Trail Creek, but also trapped and stored sediment — both pre- and post-fire. With the increased sediment load, the beavers abandoned the area. This led to breaching of dams during high runoff events and subsequent debris flows, which caused severe bank erosion and channel realignment in some locations.
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| Figure 3. Sediment Yield Calculations for Trail Creek |
Erosion and Sedimentation
Significant amounts of hillslope sediments have eroded into Trail Creek. The first few years following the fire, the creek showed the effects of sediment input into the stream channel. This is due to the soils in the Hayman Fire area being highly erodible when exposed to the direct impacts of rain, sheet wash, rilling, or gullying (John 2002). Field studies (E&H 2006) initiated after the Hayman Fire show that the decrease in ground cover and the resultant increase in surface water runoff led to channel initiation in formerly unchannelled swales as well as incision and gullying in existing channels.
Nearly all fires increase sediment yield, but wildfires in steep terrain produce the greatest amounts — 12 to 165 ton/acre (Robichaud et al. 2003). Sediment delivery potential in the Hayman Fire area can be based on post-fire monitoring of the Buffalo Creek Fire (Moody and Martin 2001), which demonstrated that approximately 15 ac-ft of sediment was delivered to Strontia Springs Reservoir for each square mile of burned area during the 5 years following the fire (Robichaud et al. 2003).
Calculated sediment yield rates for Trail Creek are illustrated in Figure 3. The data shows that through time, sediment yield is decreasing. In 2002, the Burned Area Emergency Response Team (BAER) estimated that the sediment yield in high and moderate burn areas was 70 tons/acre/year. This results in an average of about 41 tons/acre/year for Trail Creek. In 2003, the Forest Service estimated that sediment yield had dropped to 10 and 11 tons/acre/year within the burn area. The E & H value reported in 2005 is an estimate based on the field observations, and is not representative of any sampling. The Buffalo Creek value is based on 5 years of data and is presented only as a reference. Baseline numbers range between 0.5 and 1 tons/acre/year.
It is readily apparent that the rates:
- are decreasing through time,
- are currently lower than the 5-year average for Buffalo Creek, but
- are not yet to baseline conditions.
Recovery
The recovery of the hillslope and riparian vegetation will influence how quickly aquatic environments recover. Clearly, areas that were less severely burned will likely recover to pre-fire conditions most rapidly. Recovery of aquatic ecosystems within severely burned watersheds will be most dependent on riparian recovery, the juxtaposition to high quality habitats that can provide sources for re-colonization, and mitigation of additional chronic disturbances (Graham 2003).
Erosion rates can be expected to return to near-background levels after 4 to 5 years when the percent cover has increased to at least 60 to 70 percent. This indicates that sediment entering Trail Creek is almost near background (i.e., pre-fire) levels. Vegetation is becoming established in areas that have recently experienced significant deposition. This implies that the deposition is not inhibiting plant growth and is slowing. The remaining transportable sediment is along banks and/or on the floodplain. A majority of this sediment has already been transported downstream to the confluence with West Creek.
As noted, beaver ponds acted as sediment traps storing sediment and creating new channel banks. As the sediment is cleared out of the channel, the beavers are likely to return to their old ponds and lodges. If this happens, the beavers will stabilize the stored “floodplain” sediment.
Conclusions
Overall, the sediment yield for Trail Creek is returning to baseline conditions. This allows the riparian vegetation to recover naturally. Nature is doing most of the difficult and expensive work; removing the sediment that clogs the channels and hinders riparian ecosystems.
References
- Engineering and Hydrosystems (E&H).
- 2006. Post-Fire Sediment Yield Estimation, Trail Creek and West Creek, Douglas County.
- Graham, R.T.
- 2003. Hayman Fire Case Study. Technical Editor. USDA Forest Service Gen. Tech. Rep. RMRS-GTR-114.
- John, T.J.
- 2002. Soil Report for Hayman Fire Burned Area. Prepared for: U.S. Department of Agriculture, Forest Service, 2002. Hayman Fire — Burned Area Report. (Hayman Fire 2500-8, original 7/5/02, revised 8/21/02) Unpublished report on file at: U.S. Department of Agriculture, Forest Service, Pike and San Isabel National Forests, Pueblo, CO. 9 p.
- Moody, J.A.; Martin D.A.
- 2001. Hydrological and sedimentologic response to two burned watersheds in Colorado, U.S. Geological Survey Water Resources Investigation Report 01-4122, Denver, CO.
- Robichaud, P.; MacDonald, L.; Freeouf, J.; Neary, D.; Martin, D.; Ashmun, L.
- 2003. Postfire Rehabilitation of the Hayman Fire. USDA Forest Service Gen. Tech. Rep. RMRS-GTR-114: 293-314.