Ecological dynamics

This riparian complex is associated with alluvial fill valley (VIII) or Glacial Trough (V) valley types, which have been influenced by till and outwash. This site naturally develops a Rosgen C channel type, with potential to develop unstable G, F, or D channel types.

This site typically occurs in gentle reaches where meadows are associated with the stream. B type channels are typically present in reaches between meadows in more confined areas. Community components include a pioneer forb-mixed willow community on frequently flooded gravel bars and floodplains, a Lemmon’s willow-mixed sedge community on the floodplains, a tufted hairgrass upland grass community, and Sierra lodgepole pine community on the terraces or abandoned floodplains.

The Tahoe soil series is the dominant soil that occurs on the floodplains associated with this ecological site. The Tahoe soils have a rich surface horizon with mucky silt loam textures from organic matter accumulation. Typically there are buried soil horizons, with mucky silt loam, coarse sand, or sandy loam textures. Depth of the buried horizons varies, but the upper buried horizon is typically around 20 inches below the surface. Gleyed soil colors begin at 30 to 45 inches. The Watah soils are present in the wettest, most stable areas, where thick organic matter has accumulated. The Watah soils have 38 cm of peat over gravelly sandy loam and gravelly coarse sand. If peat dries out, due to altered hydrology, it will start to decay and become susceptible to trampling, and subsequent erosion due to water or wind erosion.

Hydrologic factors:

These streams have been heavily impacted, and historic stream conditions are difficult to find. Less impacted areas exhibit eroded C type channels, within alternating reaches of D type, F and entrenched C type channels. Beavers have impacted channel morphology in some areas.

A C type channel is a slightly entrenched single thread meandering channel with a well-defined floodplain. The channel has moderate to high sinuosity with less than two percent channel gradient. These channel types generally have a width to depth ratio greater than 12, which means they are wide and shallow. They are often found in broad valleys with well-developed alluvial floodplains and terraces. These channels typically flood over bank two years out of three. In an undegraded state, a 50-year flood event should overflow onto the floodplain. C type channels are constantly in the process of transporting and storing sediments from upstream sources or bank erosion. As the particle size of the channel bottom material decreases, the sediment supply generally increases and so does the erosion potential. The channels on this site support gravel (C4) reaches, which have very high erosion potentials, and are very sensitive to disturbance. Vegetation, such as willow and sedges exert a very high controlling influence on C type channel dynamics (Rosgen 1994).

C type channels respond quickly to disturbances, which may cause channel aggradation, channel incision, or bank erosion. This can shift a C type channel to unstable D, G or F type channels. C type channels can become unstable due to disturbances that impact stream bank vegetation, change the flow regime, or alter channel morphology. Overgrazing and excessive trampling by livestock can reduce streambank stability by reducing the vegetative cover or by 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 the wetland obligate species because the water table is lower and flow disconnected from the floodplain. The 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, 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 channel supports wetland plant communities but it is constrained by terraces to the lower floodplain where a lower water table supports a greater proportion of upland plants.

Disturbance Factors:

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). The removal of the trees increased erosion on mountain slopes and increased sediment supply to these river systems. The 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 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 Pediastrum a genus of algae, 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, forage, shade and gentle landscapes (Kie and Boroski 1996). The cattle may have affected the present composition of vegetation by selectively grazing palatable species and trampling susceptible species. Grazing can affect channel morphology by removing bank stabilizing vegetation, such as willows, and by trampling the stream banks with their hooves when accessing the stream. Fine sediment and nutrient loading (which increases algal growth) are currently the primary factors attributed to reduced lake clarity in Lake Tahoe Basin (Lahontan Water Board and Nevada Division of Environmental Protection 2008).

Road development and other infrastructure development has reduced the meander and meander belt width in several reaches, creating a stream with more concentrated flow, and scour potential.

Large flood events can cause immediate and drastic changes in channel morphology. Large areas may be subject to thick layers of sedimentation, which often cause the channel to braid. In other areas, meander bends may be cut off, a new channel created, or flows may scour the banks, eroding floodplains or terraces.

Beavers are present in these streams, but typically build dams on side channels or braided areas where flow is not concentrated. 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. Their absence may have contributed to increased erosion as historic dams decayed or washed out releasing sediment, increasing flow velocity, lowering water tables, causing channel stabilizing sedges to decline.

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.