Conceptualizing Ecological Responses to Dam Removal: If You Remove It, What's to Come?

One of the desired outcomes of dam decommissioning and removal is the recovery of aquatic and riparian ecosystems. To investigate this common objective, we synthesized information from empirical studies and ecological theory into conceptual models that depict key physical and biological links driving ecological responses to removing dams. We define models for three distinct spatial domains: upstream of the former reservoir, within the reservoir, and downstream of the removed dam. Emerging from these models are response trajectories that clarify potential pathways of ecological transitions in each domain. We illustrate that the responses are controlled by multiple causal pathways and feedback loops among physical and biological components of the ecosystem, creating recovery trajectories that are dynamic and nonlinear. In most cases, short-term effects are typically followed by longer-term responses that bring ecosystems to new and frequently predictable ecological condition, which may or may not be similar to what existed prior to impoundment.

Outburst floods provide erodability estimates consistent with long-term landscape evolution.

Most current models for the landscape evolution over geological timescales are based on semi-empirical laws that consider riverbed incision proportional to rock erodability (dependent on lithology) and to the work performed by water flow (stream power). However, the erodability values obtained from these models are entangled with poorly known conditions of past climate and streamflow. Here we use the erosion reported for 82 outburst floods triggered by overtopping lakes as a way to estimate the outlet erodability. This avoids the common assumptions regarding past hydrology because water discharge from overtopping floods is often well constrained from geomorphological evidence along the spillway. This novel methodology yields values of erodability that show a quantitative relation to lithology similar to previous river erosion analyses, expanding the range of hydrological and temporal scales of fluvial incision models and suggesting some consistency between the mathematical formulations of long-term and catastrophic erosional mechanisms. Our results also clarify conditions leading to the runaway erosion responsible for outburst floods triggered by overtopping lakes.

Geologic and hydrologic hazards in glacierized basins in North American resulting from 19th and 20th century global warming

Alpine glacier retreat resulting warming since the close of the Little Ice Age in the 19th and 20th centureis has increased the risk and incidence of some geologic and hydrologic hazards in mountainous alpine regions of North American. Abundant loose debris in recently deglaciated areas at the toe of alpine glaciers provides a ready sources of sediment during rainstorms or outburst floods. This sediment can cause debris flows and sedimentation problems in dowstream areas. Many glacier-dammed lakes have grown in size while ice dams have shrunk, resulting in greater risks of ice-dam-failure. The retreat and thinning of glacier ice has left oversteepened, unstable valley walls and has led to increased incidence of rock and debris avalanches (AU)