The impact of severe blowdown on sediment composition and sedimentation rates in lakes within blowdown patches created by the Routt-Divide Blowdown of 1997, Mt. Zirkel Wilderness Area, North-Central Colorado

The uprooting of trees due to high winds, or blowdown, is a common occurrence throughout the world. Rare, large scale blowdown events have the ability to drastically alter forest landscapes on a timescale of hours. Alone or when combined with other forest disturbances such as fires and insect outbreaks, blowdowns have the potential to modify existing watershed characteristics. Little information is known about how large-scale blowdowns affect the physical environment in general and watershed-wide sediment yield and deposition rates in particular. The few studies that have taken place have yielded inconclusive or somewhat contradictory results. Given the large scale of some events and the potential impact of such events it is important that the geomorphological effects of blowdowns be better understood, not only from a basic process but also from an applied perspective. Lakes receive and store a significant fraction of the disturbed sediment formerly found in the soil column and lake sediments have proven to be reliable and accurate records of sedimentation characteristics within their catchments. For the purpose of this study, it was hypothesized that areas subjected to blowdown will have increased erosion rates. It was also hypothesized, that lakes within blowdown areas will be subjected to accelerated sedimentation rates following events. In order to test these hypotheses and isolate the effects of a blowdown on sedimentation rate three small mountain lakes in the Mt. Zirkel Wilderness Area, Colorado were cored. The lakes selected consist of a control lake with no known recent history of blowdown in its watershed and two lakes that receive direct runoff from watersheds and slopes that were affected by a major blowdown event in 1997, the Routt-Divide Blowdown. One core was dated using the 210Pb method and multiple cores were analyzed for organic and inorganic matter concentrations to determine if changes in sedimentation rates and/or sediment compositions occurred following the event. In the case of the two lakes studied that had catchment slopes affected by blowdown, somewhat surprisingly, no significant change was observed with respect to sedimentation rates or sediment composition. In fact, sedimentation rates obtained from the dated core exhibited a decreasing trend that began decades prior to and continued through the blowdown event. This suggests that in the area affected by the Routt-Divide Blowdown it is likely that long-term changes in climate and the effect these changes have on the amount and type of vegetation present are the dominant influence on sedimentation rates, and that the Routt-Divide Blowdown of 1997 did not increase the rates. Based on further evaluation of the sites and the event it appears that slope characteristics and treefall orientation may be significant factors controlling the amount and type of disturbed sediment that moves downslope. In the case of future large-scale blowdowns, where their effect on water quality or sedimentation rates is in question, dominant treefall orientation with respect to slope should be an integral part of initial assessment, especially in remote area, and be considered when interpreting data from any similar study.