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Post-Fire Watershed Recovery: Update to Trail Creek Case Study

Brian Murphy, PE, Engineering & Hydrosystems, Inc.

Figure 1 Trail Creek Watershed Conditions (October 2006)

Pre-Fire Watershed Characteristics
The Trail Creek watershed is approximately 11,000 acres of forested land in Douglas and Teller Counties. Trail Creek runs the length of the watershed with steep hillslopes (up to 25%) connecting the narrow valley. The watershed experiences significant grussification (i.e., the granite bedrock is chemically weathered near the surface). The watershed has multiple roads that run through it but is not altered in any other notable fashion. Prior to June 2001, the watershed was a heavily forested area with stable hillslopes, tributaries and riparian corridor. Trail Creek experienced little sedimentation, bank erosion, or scour.

Post-Fire Watershed Characteristics
The Hayman Fire changed the watershed characteristics dramatically. The 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. Decreased infiltration rates increased peak flows, runoff, and erosion. The severity of increased runoff and erosion is mainly determined by burn intensity and vegetation recovery and to a lesser extent by slope.

The USFS did extensive burn severity mapping of the Hayman Fire burn area including the Trail Creek watershed. Burn severity was used as the main attribute to identify priority restoration areas.

Current State of the Watershed
Since 2002, large portions of the watershed have recovered naturally or with the assistance of post-fire rehabilitation treatments. A GIS analysis of aerial photographs taken in October 2006 show 67% of the Trail Creek watershed is in good condition (Figure 1). Good condition is defined as 60-70% vegetation cover and limited tributary channel sedimentation. Pre-fire vegetated cover is approximated at 80% on hillslopes and close to 100% in the valley riparian areas.

33% of the watershed has been slower to recover and has poor hillslope and tributary conditions. Poor condition is defined as less than 50% vegetation cover and extensive tributary channel sedimentation. Much of the watershed in poor condition is in areas classified as moderate and high burn severity.

Figure 2 Hillslope in good condition (60%-70% vegetated cover), June 2007.

Hillslope
Given that the Hayman Fire occurred over five years ago, Trail Creek with 15% of its total watershed in unburned area and 39% in a low burn severity area (1-2 year recovery) is well on its way to pre-fire hillslope erosion rates. Vegetation on the hillslopes is well established in these areas (Figure 2). However, significant hillslope erosion still occurs in moderate and high burn severity areas where ground vegetation is slower to return. This active hillslope erosion causes tributary channel sedimentation. During large storm events, stored sediment in tributary channels is transported to the mainstem of Trail Creek.

Riparian Areas
Tributaries
Over the course of five years, significant amounts of hillslope sediments have eroded into tributary channels (Figure 3). The areas of most tributary channel sedimentation are located in moderate and high burn severity areas. Headcuts are present in a number of these tributaries. This means that degradation in these tributaries is active and sediment transport into Trail Creek will continue. However, the natural recovery of hillslope vegetation has gradually reduced the volume of eroded sediment transported into tributaries. Many of the tributaries in the upper portion of the watershed are in good condition (Figure 1) with beaver ponds and dams, destroyed during flood events after the fire, rebuilt by returning beavers (Figure 4).

The hillslope and tributary delineations depicted in Figure 1 were field verified in June 2007. Field evidence indicates that hillslope erosion has decreased dramatically since many of the main tributaries are showing signs of returning to original base sediment levels (supply limited). Head cuts have formed at junctions between tributaries and the mainstem and are proceeding in an upstream direction.

Figure 3 Tributary Stream to Trail Creek with significant sedimentation and the start of a headcut.

Figure 4 Rebuilt beaver dam in the upper portion of the Trail Creek watershed, June 2007.

Trail Creek
The condition of Trail Creek varies depending on vegetation recovery on the hills and tributary channel sedimentation. In the upper portion of the watershed, the hills and tributaries are in good condition; therefore Trail Creek is in good condition. The upstream portion is much less impacted by major runoff and sedimentation following the Hayman Fire compared to the lower portion. This is due, in part, to the post-fire USFS treatment of mulch to the ground. The upstream portion also had more unburned and low burn severity areas which generate less transportable sediment to Trail Creek. Hence, channel downcutting and bank erosion are less pronounced. In-channel beaver dams also helped control flood discharges that produce large sediment transport capacity. Beavers are much more active in the upstream portion.

Recovery of the lower portion of Trail Creek is much slower, as the tributary channels are incising, and the Trail Creek channel is aggrading due to the large amount of incoming sediment. The in-channel sediment load takes longer to return to pre-fire yields. However, Trail Creek is confined by the valley and the road through most of the reach. Therefore, there is little volume available for storage of sediment throughout most of the main channel. The main area of concern is the shallow-gradient valley just upstream of the confluence with West Creek.

A large storm event in July 2006 caused increased hillslope erosion in some areas adjacent to the creek resulting in large sediment input into the Trail Creek channel. This indicates that moderate and high severity burn areas in the lower portion of the watershed are still actively eroding. No significant erosion occurred in the upper portions of watershed.

Watershed Restoration
Restoration of the Trail Creek watershed is multipurpose. Restoring the watershed is a natural method to control erosion and sediment transport while returning the land surface and streams to pre-fire ecological conditions. Hillslope restoration is a straightforward task of reestablishing vegetation through seeding and mulching. Stabilization of the tributaries and Trail Creek is a more difficult task and requires more structural techniques than the hillslope restoration.

A strategy to control the source of sediment through hillslope restoration and secondarily deal with the existing sediment problems on Trail Creek is being developed. This follows standard of practice to manage high sediment loads by controlling them at the source (Goldman et al. 1986). However, the problems on Trail Creek need to be addressed as well and therefore secondary measures for in-channel restoration will be considered.

Conclusions
Erosion is a major concern in the Trail Creek watershed. Focusing on hillslope erosion addresses the root cause of sediment entering Trail Creek. Areas where the hills are in good condition (i.e., 60-70% vegetation ground cover), the tributaries are also in good condition, as is Trail Creek. Alternatively, if in locations where the hills are in poor condition, the tributary channels are most likely experiencing active sedimentation that is transported down-valley into Trail Creek. Restoration of the Trail Creek watershed should focus on establishment of hillslope vegetation in high-priority hillslope restoration areas. Secondary measures for in-channel restoration are also necessary in strategic locations.

References
Goldman, S.J.; Jackson, K.; Bursztynsky, T.A. 1986. Erosion and Sediment Control Handbook. McGraw-Hill, Inc. San Francisco.