Frigid Sandy Flood Plains
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition T1A More details
- Restoration pathway R2A More details
- Transition T2A More details
- Restoration pathway R3A More details
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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Description
This state is a single thread B type channel found on the deep, coarse volcanic deposits erupted from Lassen Peak or in alluvium. B channels are considered moderately entrenched and have a moderate to high width to depth ratio. Sinuosity is moderate, resulting in a riffle-dominated channel with infrequently spaced scour pools. B type channels are sediment-limited systems but this site supports unstable gravel-bottom (B4) and sand bottom (B5) reaches that have high erosion potential. The sensitivity to disturbance is high for both B4 and B5 types and vegetation exerts a moderate controlling influence on stream dynamics.
Obligate wetland species dominate the site in the undisturbed community phase. A shrub community of thinleaf alder/ willow is found on the stream banks of the active channel and the floodplain. A pioneer plant community of native herbs is found on recently exposed substrate, generally along the stream channel, but may also be found on upper side channels. The adjacent upland habitat is coniferous forest of Sierra lodgepole pine.
Submodel
State 2
Stream Channel Succession
Description
B type channels can become unstable due to disturbances which impact the stream bank vegetation, change the flow regime, or alter the channel morphology. Overgrazing and excessive trampling by livestock can seriously reduce streambank stability. Additional stream classification data is required to determine which channel types are expressed as the B channel evolves in response to disturbance. Continued disturbance can entrench the stream. The Rosgen Stream Succession Scenario #6 diagram is displayed below. In this possible succession scenario, the “B” type channel in State 1 down-cuts into an entrenched low gradient “G” type channel that naturally begins to widen over time into an entrenched “Fb” type channel. The plant community components associated with these possible successional channel types is not known. However, deeply incised “G” and “Fb” channels generally lose the wetland obligate species and become dominated by upland grass, shrub and/or forest plant communities. As the water table lowers at the site, the adjacent conifer community of Sierra lodgepole pine and white fir would encroach and reduce the thinleaf alder and mixed willow riparian community. Upland grass species like blue wildrye and western neeedlegrass would begin to dominate the native herbaceous community. Eventually, a new entrenched “B” type channel may form that resembles the original “B” type channel. The entrenched channel still supports some wetland plant communities but it is constrained by terraces which have a upland plant communities.
Submodel
Description
The constrained B Channel on this ecological site has further downcut through the deep, coarse volcanic deposits erupted from Lassen Peak. As sediment deposition in the broadly entrenched “F” type channel continued, the vegetation increases channel stability and eventually channel sinuosity returns and a new meandering channel will develop that has similar morphological features of the original B channel but is constrained by the old floodplain, which is now a hydrologically disconnected dry terrace.
The obligate wetland species that dominate the site in the undisturbed community phase, have been reduced under the drier conditions, so that the riparian shrub community of thinleaf alder/ willow along the stream banks of the active channel now occupies only 20% of the total vegetation. The pioneer plant community of native herbs persists on recently exposed substrate, but may have more non-natives and a greater proportion of upland grasses than sedges. The adjacent upland Sierra lodgepole pine and mixed conifer forest is now the dominate vegetation.
Submodel
Mechanism
This transition can occur naturally as the “B” type channel in State 1 down-cuts through the deep coarse volcanic deposits erupted from Lassen Peak. The channel on this site supports gravel-bottom (B4), and sand bottom (B5) reaches which have high erosion potentials. Bank erosion is a natural river adjustment process and can as the result of mass wasting, liquification, freeze-thaw, fluvial entrainment, and ice scour. The coarse colluvial and alluvial deposits in the valley bottom are gradually eroded through these processes.
This transition can be initiated by a disturbance that alters the hydrology of the site including channel realignment and/or confinement, culvert installations, and road construction. Such alterations straightened the channel, eliminating its ability to meander. This increases the shear stress along the stream bank, which accelerates stream bank erosion, thereby increasing sediment supply and decreasing sediment transport capacity. As the channel straightens it has a shorter course down the valley, creating a steeper gradient. The stream may adjust by headcutting into the base level of the channel until it establishes a new gradient through the meadow. As the stream bed is lowered, so is the water table in the meadow. The new larger and steeper channel can contain more flow, reducing the frequency of flooding and the extent of the floodplain, creating terraces.
Disturbances such as cattle grazing and/or the seeding of non-native grass for forage that impacts the vegetation along the stream bank can cause a similar entrenchment of the channel but through a different mechanism. When cattle reduce vegetation and trample the exposed stream banks, the resulting erosion also leads to eventual straightening and headcutting of the channel. Likewise, when non-native grasses displace the native riparian sedges and rushes, the stabilizing root mats that help prevent erosion are also removed.
Mechanism
Additional stream classification data is required to determine which channel types are expressed in State 2 as the B channel evolves in response to disturbance. Without this data it is not possible to identify the restoration pathway for the intermediate phase of the stream succession scenario.
Mechanism
This transition occurs when continued sediment deposition in the broadly entrenched “Fb” type channel allows riparian vegetation to reestablish on the new floodplain. The vegetation increases channel stability and eventually channel sinuosity returns creating a new “B” type channel that is more entrenched and constrained than the original “B” channel.
Mechanism
The processes that have altered this stream system may be natural or human-influenced. If the primary reason for channel adjustment is a result of natural erosion through the debris or glacial deposits, the channel should be fairly well readjusted and left alone. However, if human caused disturbance has caused the erosion, then restoration procedures should be considered.
The goal of restoration is to return the channel to its full potential, which is defined as the best channel condition, based on quantifiable morphological characteristics of that stream type. For a constrained B channel the goal is to raise the water table, increase the channel sinuosity, and re-establish riparian vegetation on the stream banks to reduce bank erosion. Each segment of the stream channel should be surveyed to determine the evolutionary stages of channel adjustment and evaluate the potential for natural recovery. If natural recovery does not seem likely, a thorough stream departure analysis can determine the feasibility of restoration, anticipate response to future changes in management, and develop appropriate restoration designs.
Model keys
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