Leaky Rivers: Nutrient Retention and Productivity in Rocky Mountain Streams Under Alternative Stable States

Research Project: 

RB00CM8.14.2

Project Manager: 

David Walters
Leaky Rivers: Nutrient Retention and Productivity in Rocky Mountain Streams Under Alternative Stable States. Photo by Michael Venarsky, USGS.
Leaky Rivers: Nutrient Retention and Productivity in Rocky Mountain Streams Under Alternative Stable States. Photo by Michael Venarsky, USGS.

Streams of the southern Rocky Mountains (and elsewhere) suffer legacy effects of beaver trapping, wood removal, timber harvest, log floating, and other activities that have greatly reduced the size and along-stream frequency of logjams. Such streams are “dam-impoverished” ecosystems with greatly reduced capacity for organic matter storage and processing, and thus exhibit effects opposite to rivers where dam construction has greatly altered stream structure and function. Subsequently, the jam loss has reduced the productive capacity of headwater stream networks, reducing their ability to store nutrients and to grow animal biomass (secondary production). These responses are not linear. Rather, these streams exhibit alternative stable states. Wood recruitment to streams fluctuates through time due to natural processes (e.g., wildfire) and human disturbances (e.g., timber harvest) but the retention of wood within streams, does not necessarily correlate with recruitment rate. Wood that enters a stream channel is more likely to be retained if there are closely spaced obstacles that interrupt its downstream movement. There is a positive feedback such that high wood loads trap more wood.

The goal of this research is to understand how river geomorphology, log jams, and land use affect in-stream nutrient cycling and animal production. A few large (>20 km2) patches of old-growth forest remain in Rocky Mountain National Park, providing a unique opportunity to assess pre-colonization frequencies of logjams in mountain streams and to characterize stream responses to logjams under “reference” conditions. Our objectives are to measure the effect of wood and wood loss on ecosystem metabolism and nutrient-cycling as well as stream and riparian animal composition and secondary production. We will then scale these site-specific measurements to entire watersheds. Combined, these objectives will allow us to examine the legacy effects of stream management histories on ecosystem processes, to link physical habitat to animal conservation and ecosystem function, and to provide the first landscape-scale assessment of the effects of jam removal on stream processes. In short, this study will provide the most holistic understanding of how management history has influenced mountain stream ecosystem dynamics to date.