History of Man-Made Alterations
The Upper Truckee River drains a 54 square mile watershed, and is the largest tributary and contributor of sediment and algae-inducing nutrients flowing into Lake Tahoe . The watershed receives from 25 to 60 inches of precipitation a year, primarily in the winter months.
The river starts high in the Tahoe National Forest in the Meiss Meadow area and draws water from Dardanelles Lake .
After reaching the developed neighborhoods at Christmas Valley , the river's health begins to deteriorate. In addition to current impacts, the river bears the scars of years of disturbance within the watershed and to the river itself. Impacts from erosion, pollutants and loss of natural wetlands are the result of construction of homes and businesses along the river, large bridges such as on Highway 50, the Lake Tahoe Golf Course, the Lake Tahoe Airport , direct river straightening, historic and current grazing and construction of the Tahoe Keys.
The result is an increased flow of fine sediment, nitrogen
to Lake Tahoe , which contributes to algae
growth and loss of water clarity.
The period of Comstock logging, from about 1860 to 1890, resulted in widespread land disturbance that likely affected the river and surrounding watershed. Several direct modifications of channels occurred during this period, including the construction of small dams to run mills and splash dams to provide water to float logs down the river. Splash dams were generally 10 to 20 feet high timber or earthen structures designed to create small ponds or lakes.
They were designed to be destroyed at the height of runoff in the spring, typically with explosives. The resulting flood helped carry logs to the lake. The highly sinuous historic channel probably didn't float the logs very well. Crews likely had to dredge and modify the channel to salvage logs that got caught up and remove natural log jams to improve channel transport efficiency.
It is likely that meanders were straightened to improve log-transport efficiency. These operations increased erosion of the channel and had severe impacts on habitat.
Comstock logging also had pervasive watershed effects. Loss of trees and compaction of soils led to increased runoff. Soil disturbance resulted in increased erosion and sediment supply.
Ranching Operations and Flood Control
All meadow areas have been grazed since the latter part of the 1800s. The primary type of grazing has been cattle for dairy operations, although some grazing for sheep and horses also occurred. In some areas, the channel was probably modified to reduce the impacts of floods on adjacent pastures (small levees, meander cut-offs).
Channelization to improve agriculture operations and decrease flooding has occurred through the lower reach of the river. Within the project area, the river was straightened just prior to these 1940 photos.
Dredging and rip-rap have occurred in several locations since that time. Woody debris was probably also removed to improve flow capacity.
Adjacent meadows and in feeder streams were modified either for drainage or irrigation prior to 1940. Angora Creek has been highly modified in the area of the confluence with the river. T his reach of Angora Creek was previously restored by State Parks. The U.S. Forest Service has also completed a restoration project at Cookhouse Meadows.
Modern development led to further modification of the lower river. The Tahoe Keys development in the early 1960s occupied a substantial portion of the delta and marsh at the lower end of the system. The river was channelized along the margin of the development, leading to loss of the delta system of the river. This would have been the natural area of sediment deposition for the river, instead of carrying the material into Lake Tahoe.
Highway 50 bridges at South Upper Truckee Road and at the Elks Club Drive substantially altered the channel and floodplain. Floodplain area was significantly reduced, and the channel cross section was narrowed.
The Lake Tahoe airport was constructed in the lower meadow reach in the early 1960s. Almost entirely within the valley flat, the airport occupies most of this portion of this meadow. The river was channelized along the eastern portion of the meadow, including substantial grade modification to control incision.
The Lake Tahoe golf course was constructed within the river floodplain between 1959 and 1964. Several of the holes occupy former meadows and floodplain directly adjacent to the channel. In the lower portion of the project area, the golf course essentially borders the river, with little or no riparian vegetation remaining. Five bridges have been constructed across the river within the golf course. Because the bridges were generally undersized, most have required extensive maintenance to control local erosion.
Urban related development accelerated in the 1960s with a myriad of housing and roads throughout the watershed. Gravel mining of outwash terraces provided gravel for construction, including the borrow pits in Washoe Meadows State Park (WMSP), one of which has been restored.
Sewer lines were installed.The sewer is a gravity system so it occupies the valley flats and floodplains, impacting drainage patterns and habitat. The sawmill reach of Angora Creek was captured by the sewer excavation and the stream incised a new channel directly over the sewer line. This reach was also restored by State Parks.
River Channel Alterations
Because many of the impacts of European land use practices took place prior to photos or other records of the condition of the channel, the effects must be inferred rather than directly measured.
The land use history has resulted in a substantially altered channel. Channel length and sinuosity (curviness) have been reduced throughout the lower river. In the project area, there has been a 28 percent reduction in channel length between 1940 and 2004, and several meanders appear to have been directly cut-off in the 1940's aerial photo. Similar impacts occurred in broad meadows downstream to the lake.
Straightening stream courses tends to accelerate water flows and cause bank erosion, which increases transport of fine sediments into Lake Tahoe. The mechanical process of straightening streams often results in the loss of wetland areas, which have the capacity to filter pollutants from the watershed.
In many locations, the channel has also been enlarged. Channelization was designed to drain surrounding valley flats and provide flood protection, and the new channels that were constructed were therefore likely bigger and deeper to provide more hydraulic capacity.
The river no longer connects with its floodplain on a regular basis. Fine sediment which would normally deposit on the floodplain is carried downstream.
RIPARIAN ECOSYSTEM IMPACTS
Riparian Vegetation Impacts
Direct disturbance has altered many of the riparian vegetation communities in the lower river.
Grazing in larger riparian meadows altered historic riparian vegetation, and changes in geomorphic processes resulting from channelization have impacted remaining riparian vegetation communities.
Incision throughout the lower river has resulted in lowered groundwater tables in meadows. Although the magnitude of incision has been relatively small (probably from two to five feet in most areas), the ecological consequences of lowered groundwater levels have been more substantial.
In wet or mesic meadow communities, the majority of the root zone is within about two to three feet below the ground. Changes of only one or two feet in groundwater level are sufficient to change wet communities to mesic types. Lower groundwater tables have also shortened the growing season and generally reduced riparian plant productivity.
Although natural rates of streambank erosion promote shrub recruitment and vegetation community patchiness, incision has greatly increased the erosive stress and scour placed on streambanks during floods. Streambanks are also higher, with less available moisture.
As a result, riparian shrubs are less capable of colonizing streambanks disturbed during floods, and streambank instability is common throughout the lower river. Riparian bands along the channel, particularly throughout broad meadows, have been eliminated or reduced in size.
Streambanks and Erosion
Incised channels are more capable of transporting bedload and tend to export bedload, rather than storing it in bars within the channel. Processes of channel migration have also been slowed by incision and, in many locations along the river, meander migration has been eliminated through rip-rap or other channel constraints.
All of these changes have reduced the development of instream bars, particularly point bars, which are focal points for riparian shrub recruitment. A loss of in-channel bars has reduced the area of riparian shrubs. This impact is prevalent throughout broader meadows, but far less pronounced in reaches entrenched in outwash, where fluvial adjustment to incision has led to the development of extensive in-channel bars, and distribution of shrubs is widespread.
In summary, human modifications of the floodplain and stream channel and subsequent channel responses have had important consequences for riparian vegetation communities. Specific effects on riparian vegetation include the following:
Reduced frequency and extent of establishment of new individuals of the dominant, community-defining species;
Lower vigor (or death) and productivity of surviving individuals, especially of species that are not as tenaciously drought-adapted as C. nebrascensis or J. balticus;
Overall conversion of wetter habitats to drier ones, and loss of small areas of standing water that are of great wildlife habitat value;
Reduced woody riparian habitat connectivity;
Lowered overall habitat complexity due to the ultimate loss of individuals from previous establishment episodes and lack of new colonization, resulting in lowered habitat values for wildlife;
And consequent loss of diversity and lowered resistance to future catastrophe, disturbance and/or gradual environmental change.
Historically, the river was the most important fishing resource for the Washoe tribe.
The Washoe fishery focused on spawning fish from Lake Tahoe , especially Lahontan cutthroat trout and whitefish .
Based on accounts from Washoe elders, Lahontan cutthroat spawning was most extensive in the middle reaches of the lower river, from about the upstream end of the project area to Meyers.
Maps of preferred Washoe fishing spots focus on this area, where the gravel deposits preferred by salmonids for spawning were likely extensive. Gravel recruited locally from outwash deposits and sorted and stored by channel processes including interaction with woody debris likely provided complex and high quality aquatic habitat.
These accounts suggest that the historic habitat throughout the lower river was comprised of abundant pools and gravel riffles. Intact riparian vegetation likely provided extensive cover and allowed for undercut streambanks.
Temporary development of a lagoon at the mouth of the river and a complex system of distributary channels immediately upstream were important rearing habitat for juvenile fish migrating back to Lake Tahoe .
Channelization and resulting incision have dramatically altered aquatic habitat. The direct result of channelization is simplified habitat, with the loss of pools and riffles. Riparian cover was eliminated during channelization, and the high streambanks resulting from subsequent incision do not allow for the development of overhanging cover or undercut streambanks.
Loss of Instream Cover, Deep Pools
It is also likely that woody debris is less abundant throughout the system, especially in reaches entrenched in outwash where lodgepole forests border the channel. Reduction of woody debris has had important consequences for aquatic habitat.
Perhaps the single best descriptor of functional aquatic habitat is high complexity; reduction of woody debris significantly reduces complexity. This has led to less instream cover, fewer and shallower pools, less storage of gravel substrates important for salmonid spawning or macroinvertebrate habitat, fewer backwater and marginal habitats and reduced refugia during floods.
Subsequent changes in geomorphic processes due to the incision response have further exacerbated habitat degradation. Much of the channel margin, particularly in low gradient meadows, consists of resistant lacustrine deposits. Gravel bedload is effectively transported over this material rather than being stored within the channel as bedforms.
Gravel riffles, the required substrate for salmonid spawning, are now relatively rare, especially in meadow reaches, to the extent that the California Department of Fish and Game undertook a limited project to improve spawning habitat upstream of the project area, probably in the 1960s.