Runoff and Streamflow
When precipitation falls to the earth's surface, it has to go somewhere. While a majority of it evaporates back into the atmosphere, some of it travels across the surface as runoff. Some of that runoff infiltrates into the soil, where it is used by plants. Some infiltrates even farther down, into the water table, where it becomes groundwater. Still some of that runoff ends up in streams, where it contributes to surface streamflow. In the Beaver Creek watershed, 98% of precipitation leaves the watershed as evaporation, transpiration, or surface streamflow (Baker, 1982).
In southwestern streams such as Beaver Creek, steady, low-energy winter precipitation can significantly increase base flow in streams. In contrast, summer precipitation is typically more episodic, resulting in higher energy flows with shorter durations and, consequently, more erosive power. Spring floods occur when temperatures begin to rise and snow is replaced by rain. The rain helps to melt the snow, which increases runoff during these storms. These floods can strip vegetation along stream banks, erode stream banks and channels, and carry large volumes of material downstream. As the velocity of flow decreases, floodwaters deposit these large volumes of material in a short period of time. The flooding process thus results in major changes to a stream channel.
A significant portion of the perennial baseflow in Wet Beaver Creek is spring-fed. Springs discharge from regional aquifer units such as the Coconino sandstone and the Supai Formation, or from locally perched aquifers in Kaibab Limestone, volcanic rocks, or alluvium. Recharge to the regional aquifer units such as the Coconino, Supai, and Kaibab occurs on the Colorado Plateau. Thus, a large volume of Beaver Creek surface water is derived from groundwater that originates on the Colorado Plateau.
Channels caused by water erosion in the Coconino Sandstone, Wet Beaver Creek Canyon
As runoff travels along the earth's surface, it facilitates the erosion of soils and sediments. Nevertheless, under some circumstances, erosion can be limited by soil types and conditions. In general, lack of vegetation facilitates the erosion process; but soils that have been hardened by high-intensity fire or that contain a large percentage of coarse rock fragments, for example, tend to be resistant to erosion--regardless of vegetation conditions. There are several different categories of erosion, including: channel erosion, soil erosion, sheet erosion, mass wasting, and others. Erosion can occur slowly, rapidly, suddenly, or gradually.
Erosion is arguably the most powerful geologic process at work on the earth's surface. The amount of erosion that occurs in any one location over a given period of time is a function of several variables, both natural and human. Erosion is influenced by other geologic processes, by vegetation density, by climate, by gravity, by fire, and by land use and management practices. The human factors that affect erosion can often be the most influential, as they can also alter natural processes. Effectively, increased rates and/or volumes of erosion can disrupt the equilibrium of riparian areas within watersheds like Beaver Creek by creating a sediment imbalance.
Tectonic uplift, faulting, and fracturing are the most prominent geologic facilitators of erosion. All three can contribute significantly to accelerated rates of erosion. Both uplift and faulting can result in a steeper stream gradient, encouraging more rapid stream incision. Faults and fractures can facilitate the movement of water along a given path (the fault or fracture), encouraging more rapid stream incision. Additionally, mass wasting events (landslides, mudslides, rock slides, etc.) move large volumes of sediment and/or soil in short periods of time; however, mass wasting is not a major issue in the Beaver Creek watershed.
Vegetation type and density are often determined by land use practices, many of which can be detrimental to the health of the watershed. Vegetation helps reduce erosion by creating a cohesive network of roots that help to make soil more cohesive. Vegetation also reduces the direct impact of raindrops on soil particles, which are dislodged by the action of rain.
As mentioned above, the dominant flood-drought cycle in southwestern watersheds results in episodic flooding that can erode massive quantities of soil under certain soil and vegetation conditions.
Gravity is a force that is always at work on the earth's surface. Soil and sediment on hillslopes are particularly prone to erosion by gravity, especially when slopes become saturated during rainstorms, and even more so in the absence of vegetation.
Fire can result in hydrophobic soil layers that are highly resistant to erosion. However, the resistant soil layers are also resistant to water infiltration, or hydrophobic. Hydrophobicity usually develops just below the uppermost soil layers, creating a hard surface for water and debris to flow over. As a result, high precipitation levels following a fire can result in catastrophic flooding and mass wasting events on hillslopes.
Land Use and Management
As mentioned above, removal of vegetation can increase erosion significantly. Lands that have been used for grazing purposes are often cleared of brush, lending to a greater potential for erosion.
In general, sediment yields increase when vegetation is removed from an area. Studies of sedimentation in the Beaver Creek watershed have found no appreciable difference in sediment yields between normal conditions and those in which vegetation has been removed (Thorud and Ffolliott, 1973; Clary et al., 1974; Neary and Hornbeck, 1994; Robichaud et al., 2000). According to Neary and Hornbeck (1994), the greatest sediment yields produced in areas where vegetation was removed and intense storms occurred were only six times the background erosion levels. Nevertheless, six times the background erosion levels could be significant, especially if more and larger areas had vegetation removed. Thorud and Ffolliott found that over 50% of sediment is generated during the winter in areas where vegetation has not been removed by human activity.
Suspended sediment concentrations in the drainages of the Beaver Creek watershed were found to be much higher in areas where vegetation had been removed, but still relatively low compared to other southwestern watersheds. This is due in large part to the relatively hard volcanic soils found in the watershed, limiting sediment supply. It has also been attributed to the low-energy storms during the winter months that are responsible for the largest percentage of precipitation in the area ( Lopes and Ffolliott, 1993; Baker, 1999).