THE EFFECT OF SUB-ALPINE LANDSLIDES ON HEADWATER STREAM GRADIENT AND POSSIBLE SITES FOR BEAVER INHABITATION

During our previous work in the southeastern San Juan Mountains, we noted that headwater streams on landslides were typically characterized by the presence of beaver dams whereas other headwater tributaries typically lacked beaver dams. This trend led us to hypothesize that 1) beaver tend to build beaver dams in headwater streams on landslides because the hummocky microtopography of landslides provides ideal locations for dam building and 2) beaver preference of landslides would be visible either in overall stream gradient or in minimum stream reach gradient. In order to test our hypothesis, we visited 10 landslide sites within the field area and collected data including location, elevation, evidence of beaver activity, vegetative type, and surface morphology. In addition, we visited adjacent headwater streams that showed no evidence of previous landslide activity. We then compiled data about beaver presence and absence on and off landslides. This dataset was then statistically analyzed using the G-test, a statistical test that assesses whether or not variables are independent of one another . We were able to reject the null hypothesis that beaver presence is independent of landslide location with a significant confidence interval (P=.003) indicating there is very likely a relationship between beaver presence and landslides. Our secondary hypothesis led us to further examine the profiles of all headwater streams visited in the field. The results suggested that while the average slope of landslide streams is lower than the average slope of other headwater streams, the individual gradient values are too similar to be predictive. However, it is clear that beaver prefer lower gradients and landslides tend to have lower gradient. Thus, the observed distribution of beaver dams is logical and is likely the result of the generally lower gradient of streams on landslides. As we begin to understand this relation, we can further our understanding of landslide topography and its effect on riparian ecosystems. Specifically, landslides may be more important to preserve because they may contain higher biodiversity than adjacent headwater stream systems.