Debris Flow/Fish

Step Wise Guide

Erosion

· Which fish bearing streams are most susceptible to debris flow impacts?

Debris flows represent one of the most hazardous erosion processes in mountainous watersheds. Debris flows are often triggered by shallow landslides and they can travel great distances along steep, headwater streams. Debris flows can impact fish habitats, roads and other engineered structures. Risk mitigation associated with road construction, timber harvest or in post wildfire environments, begins with identifying sites of potential debris flow initiation and potential debris flow deposition.

Refer to the "Warning" button in the tool interface. Follow up any remote sensing work, including using NetMap, with field work to verify environmental conditions and landslide and debris flow risk. There is never zero risk, there is only degrees of risk, which often is best considered on a relative basis.

Step 1: Go to NetMap Basic Tools > Maps > Aquatic Habitats > In drop down list, find coho, Chinook and steelhead. Values range from 0 to 1, poor to high quality habitat.

Step 2: To create the habitat values yourself, or to modify the habitat model (preference curves), Go to NetMap Aquatic Habitat Tools > Create Aquatic Habitats. Select either “Anadromous (Intrinsic Potential), Cutthroat trout (Bayesian) or Bull Trout (empirical). Follow Tech Help to create new habitat indices or to modify existing published models.

Step 3: Go to NetMap Erosion Tools > Channelized Mass Wasting > Debris Flows. In the drop down list, select “debris flow susceptibility – junctions” to determine where debris flows are likely to intersect streams that have fish habitat.

Step 4: Go to Basic Tool Module > Risk Analysis > Stream Segment Overlap. Select the environmental stressor (debris flow junctions). Select the habitat of choice (coho, steelhead, cutthroat trout). Select the proportion of risk you are interested in. For example, set the debris flow risk at the top 10%. Set the habitat quality at the top 5%. Run the tool. You may not always come up with a spatial match, if not, change the risk proportion selection (i.e., top 20% of debris flows and the top 10% of fish habitat quality).

Step 4: Once you have identified the potential overlaps between debris flows and fish habitats, Go to NetMap Erosion Tools > Generic Erosion Potential OR NetMap Erosion Tools > Shallow Landsliding and display the grid maps (either delivered or non delivered) and examine the source areas (the warmer colors) of the debris flows. By doing this, one can identify the landslide source areas that could potentially be linked to debris flow potential and their impacts to fish bearing streams.