Ecological dynamics

Abiotic Features:

This riparian complex is associated with alluvial valley types (VIII), with a reference E channel type (Rosgen 1996). The dominant soils are cryic, with deep rich A horizons. Histic soils are limited to the wettest areas along the streams, or associated with adjacent discharge slopes. These are typically first or second order streams associated with small alpine meadows. Dominant species are mountain willow (Salix eastwoodiae), Sierra willow (Salix orestera), mountain sedge (Carex scopulorum), and tufted hairgrass (Deschampsia cespitosa).

Hydrologic factors:

These small, low gradient streams meander through alpine meadows with narrow and deep E type channels. E type channels typically have thick mats of sedge roots which stabilize channel banks and help maintain a low channel width-to-depth ratio and high stream sinuosity. These small channels frequently flood onto the floodplain leaving fine deposits on the surface. The floodplains have a shallow water table that supports rhizomatous sedges and many obligate wetland species.

E type channels are often considered stable systems, but they respond quickly to disturbances, which may cause channel incision or bank erosion. This can shift an E type channel towards a G type channel. E type channels can become unstable due to disturbances that impact the stream bank vegetation, change the flow regime, or alter the channel morphology. Overgrazing and excessive trampling by livestock can reduce streambank stability by reducing vegetative cover or physically trampling the banks. Continued disturbance can cause the stream to erode and incise deeper into the alluvial sediments, containing high flows within the channel, and reducing or eliminating overbank flows onto the floodplain. The incision and stream confinement creates a deep, narrow, eroded G type channel. Overtime, the G type channel is widened by bank erosion, and a wide, entrenched F type channel develops. These channels (“G” and “F”) generally lose wetland obligate species because the water table is lower and flow is disconnected from the floodplain. Floodplains associated with G and F type channels become dominated by upland grass, shrub and/or forest plant communities. The banks are often bare due to active erosion. As the F channel obtains sufficient width, erosive scour is reduced, and vegetation can establish on the banks and along the greenline. Channel meander and point bar development occurs in the entrenched channel, allowing a new entrenched C type channel to develop. The entrenched C channel supports wetland plant communities but it is confined to the lower floodplain by the incised banks. The abandoned floodplain remains with a lower water table and supports a greater proportion of upland plants. E channels may develop within the floodplain of the C channel, as sedges continue to capture fine sediments and stabilize banks.

Disturbance Factors:

These upper elevation meadows are generally less disturbed than the lower elevation meadows around Lake Tahoe. The main disturbance in higher elevation meadows is from historic grazing. Grazing has altered the composition and cover of the native vegetation, and caused changes in channel morphology in some areas. More remote meadows are less impacted.

During the mid-1800’s approximately 80 percent of the forests in Tahoe were clear-cut during the Comstock era (Elliott-Fisk et al. 1996). Tree removal increased erosion on adjacent mountain slopes and increased sediment supply to these river systems. Removal of trees would also have reduced water loss to evapotranspiration and increased water flow into the streams. The side effects of the logging era on the stream morphology are unknown, but likely increased sediment supply and stream volume in these systems. Davis (, 1996) report increased erosion between 1850 and 1900 based on analysis of cores from Lake Tahoe. The erosion was attributed to logging, grazing and road development during this period. During the same period, there was an increase in the algae Pediastrum, and an increase in sedge (Carex spp.) pollen, possibly due to increased nutrient loading into the lake and tributaries.

The meadows in the Lake Tahoe Basin were grazed in the past by cattle and sheep. Over 13 dairy farms were active in the basin, and most meadows were fenced for cattle grazing. Sheep roamed the mountains, and denuded much of the forbs and grasses (Elliott-Fisk et al. 1996). Cattle concentrate in riparian habitats because of access to water and forage, shade and gentle landscapes (Kie and Boroski 1996). Cattle may have affected the present composition of vegetation by selectively grazing species such as grasses, and trampling susceptible species. Grazing can affect channel morphology by removing bank stabilizing vegetation such as willows, and by trampling stream banks when accessing the stream.

Beavers were once thought to be non-native to the Sierra Nevada, but carbon dating of old beaver dams has shown that beavers were in the Sierra Nevada since AD 580 (James and Lanman 2012, Landman et al. 2012). Beaver trapping and extirpation efforts eliminated beavers from the higher Sierra Nevada by mid-1800s. Beavers were reintroduced into the Lake Tahoe Basin from 1939 to 1949 (Beier and Barrett 1987) after 100 years or more of absence. Nine beavers were introduced from the Snake River in Idaho to the Truckee River. Since then populations have expanded to many watersheds around Lake Tahoe. In 1987 there were 0.72 colonies (3.5 beavers) per km of stream along the Truckee River (Beier and Barrett 1987).

Beavers develop dams to create deeper water for mobility, safety, and den building (Beier and Barrett 1987). Dam building raises the water table above the dam, causing pond development and flooding across the floodplain. Below the beaver dam, or after dams are removed, the channel may become unstable and scour or widen depending upon the site. The historic impact of beaver on this site is unknown, but they likely influenced the development of these streams and meadows.

Ecological sites associated with lotic stream systems are developed using channel evolution models. Stream systems are dynamic and continually evolve to reach a stable equilibrium. Streams develop identifiable stages of development based on channel morphology (Rosgen 1997). These stages are identified using a state and transition model based on stream evolution models.

The reference state is typically the pre-settlement, most successionally advanced and hydrologically stable community phase (numbered 1.1), and the community phases that result from natural and human disturbances. Community phase 1.1 is deemed the phase representative of the most successionally advanced pre-European plant/animal community including beaver activity and hydrologic conditions that influence its composition and production. Because this phase is partly determined from reconstruction and/or historic literature, some speculation is necessarily involved in describing it.

All tabular data listed for a specific community component within this ecological site description represent a summary of one or more field data collection plots taken in modal communities within the community component. Although such data are valuable in understanding the community component (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), they do not represent the absolute range of characteristics or an exhaustive listing of all species that may occur in that phase over the geographic range of the ecological site.