Ecological dynamics

Abiotic Factors:
This ecological site is a riparian complex associated with lake deltas and lacustrine fringes. The broad, low gradient valleys allow the streams to develop into anastomosed-multi-channel streams, also referred to as a Da channel type (Rosgen 1996). Beaver activity is high in these systems. Several community components (a combination of soil type and associated plant community) related to the depth of water table, micro-topography, and different fluvial surfaces are present. The dominant soils are saturated late into summer, and have developed deep organic surfaces from decomposed sedge roots. A gradation of soil and plant communities exists from the wettest areas, with sedges and organic soils to drier grass and forb communities on poorly developed sandy soils. Willows, thinleaf alder, quaking aspen (Populus tremuloides), and Sierra lodgepole pine (Pinus contorta var. murrayana) have niches within this spectrum.

Hydrologic factors:
This ecological site occurs on low gradient (typically less than 1 percent) deltas of lakes or ponds. A braided network of channels, defined as Da channel type, is the natural stream type for this system. Beaver prefer the deep channels and lake access that this site provides for mobility, safety, and den building (Beier and Barrett 1987) They are abundant and very active in this ecological site. 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. Aerial imagery from 1940 shows dendritic-branching patterns, indicating the natural branching Da system prior to that time. The branching system may have developed with historic beaver activity or from the natural tendency for channels to meander and anastomose in these river delta systems.

A properly functioning marsh system is an important filtration system. The broad, low gradient floodplain allows for suspended sediments to settle out of suspension in the slow moving flood waters and ponds. Subsurface flow is filtered as it moves through the thick organic surface. Nutrients such as phosphorus and nitrogen are also deposited with the sediments reducing the load distributed to the lake.

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).

Anastomosed channels can have high variability. They are typically deep and narrow, but wide ponds develop above beaver dams, and some channels, particularly below beaver dams may become wide and shallow. The soils are saturated to the surface for most of the summer. The dominant soils in this ecological site have a 20 to 40 cm thick histic (deep organic) horizon, composed of poorly decomposed peat, primarily from sedge roots and leaf material. Saturated conditions slow the decomposition of the plant material, allowing for accumulation of plant material over time. The thick organic material has high pore capacity and allows for easy surface flow in the upper soil, as well as filtering fine sediments and nutrients. Beaver dams may increase sediment and phosphorous retention during high flows (Muskopf 2007). Soils in higher or drier locations lack the deep histic horizon, but typically have a rich organic surface mixed with mineral soils. In some areas depositional episodes are evident by layers of buried horizons, identified by the organic layers below sandier deposits.

Disturbance Factors:

In addition to beaver activity, these areas have been impacted by dams, historic logging, grazing, road and marina development.

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 the mountain slopes and increased the 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 the stream morphology are unknown, but likely increased sediment supply and stream volume in these systems, and may have caused channel progradation (Elliott-Fisk et al. 1996). Davis (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 the access to water and forage, as well as shade and gently landscapes (Kie and Boroski 1996). The cattle may have affected the present composition of vegetation by selectively grazing 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). Development within many of these river mouths for marinas and housing have channelized and confined these steams and floodplains, creating a rapid transport system for sediments and nutrients, rather than the diffuse and slow depositional environment.

This hydrology of this ecological site is typically influenced by an adjacent lake or pond. The construction of the Lake Tahoe Dam between 1909 and 1913 raised the water level of the lake by a maximum of 6 feet (AECOM and ENTRIX 2013) above the natural rim (6,223’). Many smaller lakes in the area also have small dams. High lake levels can flood into these low marshes. The development of the Tahoe Keys has altered the groundwater gradient in the Truckee Marsh (AECOM and ENTRIX 2013) by shortening distance and increasing the hydrologic gradient from the marsh to lake level. Groundwater flows west to the marina water level rather than to the north to Lake Tahoe (AECOM and ENTRIX 2013).The consequence of these and other hydrological changes in the Truckee Marsh has been a lowering of the ground water table during dry years (when lake levels are low), and the development of an incised highly unstable F channel. Vegetation has shifted to drier upland species in many areas, and the area influenced by saturated conditions and frequent ponding and flooding has decreased.

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. However, the reference state is speculative in this case, and has been altered by conditions mentioned above, and is therefore referred to the representative state. 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 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.