Resilience Series (Part 2): Characteristics of a Resilient Watershed

Riparian areas that are composed of trees and shrubs that quickly re-sprout (i.e. aspen, willow, etc.) after disturbances, such as wildfire and floods, can reduce impacts from disturbances and aid recovery.

Watersheds have higher integrity or resiliency when they have conditions that allow them to experience disturbances and recover relatively quickly. Resilient watersheds have the following characteristics:  

Forests that are diverse in terms of both forest types and density

Forests in Colorado are disturbance dependent and the variation of forest vegetation, age and density can greatly affect the intensity and scope of a disturbance that moves through a given area. Forested watersheds that have low vegetative diversity are susceptible to insect or disease epidemics that target specific forest types. Vegetative diversity works as a buffer by insulating some of the targeted trees and also by maintaining forested conditions when only a limited number of the existing trees are affected by the particular insect or disease. Density, even if the vegetation is diverse, increases susceptibility to watershed-scale disturbances. Dense forests are usually less vigorous because the trees are competing for limited resources, which makes them more susceptible to insects and diseases. Dense forests are also more prone to burning hot during wildfires as it is easier for the fire to move through the tree crowns, rapidly consuming large amounts of vegetation. As compared to more uniformly dense forested watersheds, forests that have open areas, either of lower forest density or meadows, experience lower intensity and mixed burn severity during wildfires.

Areas of high wildfire hazard that are relatively small and separated from other watersheds that have high wildfire hazard

Many watersheds in the Upper Poudre Watershed have areas of naturally dense forest and an associated high wildfire hazard. Attempting to manage these forest types as open forest can be difficult because their natural state is to become more dense. However, if these dense areas are isolated from other dense areas, both within the watershed and compared to adjacent watersheds, disturbances that impact them will be isolated to those areas and not propagated to other dense areas, reducing the extent of the disturbance.

Intact, functional riparian areas that can respond quickly after disturbances

Riparian areas are critical in the Upper Poudre Watershed. They provide unique habitats for both plants and animals. Riparian areas filter sediment from uplands, reduce peak flows by increasing channel roughness, provide shade to reduce water temperatures, and provide organic matter to aquatic ecosystems. Riparian areas that are composed of trees and shrubs that quickly re-sprout (i.e. aspen, willow, etc.) after disturbances, such as wildfire and floods, can reduce impacts from disturbances and aid recovery.

Riparian vegetation composed of native vegetation

Riparian areas that are dominated by non-native species, such as tamarisk or cheat grass, respond differently to disturbances than native vegetation. Many non-native species respond quickly to disturbances, potentially crowding out native species that recover more slowly. Riparian areas that have even limited populations of non-native species generally face expansion of those populations following disturbances. Over the long-term, invasion of non-native species is damaging to the proper functioning of the riparian area which depends on native species for creating appropriate pathways for habitats, nutrient cycling and structure.

Floodplains are connected to streams that flood during larger runoff events

Streams can become disconnected from their floodplains due to channelization, road construction, or flooding impacts. Floodplains have important functions during peak flow events because they divert some of the flow onto the floodplain where the water slows down and drops sediment. Those overbank flows create high quality riparian habitat by providing water, soil and nutrients for plant growth. When floodplains are disconnected from streams, peak flows are higher and sediments are transported and page 2 Upper Poudre Resilience Watershed Plan Final deposited further downstream. Increased sediment transport downstream can impact water quality and results in export of soil and nutrients from the watershed, and riparian areas do not receive the sediment inputs needed to maintain a healthy riparian ecosystem.

Upland areas have appropriate ground cover, comprised of mostly native vegetation that can recover quickly following disturbances

Uplands may have minimal ground cover for various reasons including dense forest cover, or land use impacts from developments or grazing. Some forest types have reduced ground cover because the dense trees are using all the nutrients and water available at the site. Lodgepole pine, mixed conifer and ponderosa pine forest types can become sufficiently dense to have minimal ground cover. In areas with minimal ground cover, recovery following disturbances is slower. This not only increases the potential for erosion and soil loss but also gives non-native species a competitive advantage, as these species tend to react more quickly than native species to disturbances.

Roads that have minimal impacts on watershed functions

Roads create impervious ground and require drainage systems to control runoff. Roads can increase runoff, peak flows and sediment yields to streams. Adequately designed drainage systems can minimize these impacts. Roads on the granitic derived soils in many places in the Upper Poudre Watershed have proven to be especially susceptible to increased runoff and sediment yields. There are many roads that are located in riparian areas or floodplains adjacent to streams. Roads next to streams have a higher potential for impacting streams because they reduce the size of riparian areas and floodplains, and the proximity increases the risk of sediments impacting the stream. Road/stream crossings can also be undersized, which is of particular concern following disturbances. Post-wildfire runoff usually carries high debris loads and has higher peak flows, which can overwhelm an otherwise functional culvert.

Where development occurs in watersheds, it has minimal impacts on watershed functions

Developments create concentrated areas of impervious ground that increases runoff and peak flows. Runoff from developed areas often contains pollutants; such as sediment, oil, nutrients, etc. Developments create higher road density, which increases runoff, peak flows and sediment yields. In rural areas, developments may create a high density of septic systems, which can impact water quality if they are close to streams.