Ecological dynamics
This willow riparian site complex is composed of several distinct plant community components which are adapted to various channel and bank configurations based on the depth and duration of seasonal surface and subsurface flows. The site occupies three riparian zones, the toe zone, bank zone and over bank zone. Within the toe zone, silty point bars are initially dominated by sedges and bulrush while narrowleaf willow, a rhizomatous species, occurs on gravelly bars. The extent and duration of surface flows and groundwater have a major effect on site composition and production of these three plant community components.
Narrowleaf willow is very flood tolerant, very frost tolerant and drought resistant (Anderson 2006). Narrowleaf willow may respond to fluctuating water tables and floods by increasing secondary root growth and increasing root elongation. Vegetative sprouting from roots is an important form of reproduction and narrowleaf willow can often form large clonal colonies. Following fire, narrowleaf willow will often resprout from roots, root crowns and basal stems. As a flood adapted species, narrowleaf willow is adapted to continue growing through recently deposited sediment or reestablish where broken twigs are redeposited. Narrowleaf willow can be an important source of browse for livestock. One study in Oregon found combined utilization on gravel bars ranging from 27 to 48 percent for yellow and narrowleaf willow (Kauffman et al. 1983). It is also browsed by elk and heavily used by beaver. Narrowleaf willow’s rapid growth and spreading habit make it ideal for restoring many riparian areas. Heavy livestock grazing may reduce the density of willow in riparian areas and trampling may reduce establishment. It may re-establish quickly following grazing release if it has not been severely degraded (Anderson 2006).
An obligate wetland plant, reed canarygrass may reproduce rapidly through spreading rhizomes which may form a dense sod (Waggy 2010). This habit also allows the species to regrow following the removal of aboveground biomass by low to moderate severity fire, mechanical means or herbivory. Reed canarygrass can also spread into new sites from seed, which may form a soil seed bank that can resist flooding. Growth may be reduced by shading and can be severely reduced beneath an overstory canopy, such as that created by mature willow or riparian tree species. While there is debate on whether reed canarygrass is native or non-native to North America, invasive populations of non-native strains or hybrids are widespread in the northwestern US (Waggy 2010). Reed canarygrass has been associated with decreased biodiversity in many wetland and riparian habitats. Additionally, the species may alter hydrologic flow of streams by forming a dense thatch and increase deposition by collecting entrained sediment (Waggy 2010).
Historically, the ecological dynamics of the site would have been influenced largely by climate cycles affecting seasonal runoff, droughts and flood. These processes would have been partly controlled by the type and cover of upland vegetation throughout the watershed which would have modified water capture, storage and sediment loads. These upland dynamics would have been altered by historical fire regimes and subsequently vegetation succession, erosion and runoff.
When the condition of the site complex deteriorates as a result of improperly managed grazing, willows rapidly decrease along with palatable grasses and sedges. Willows are severely impacted by heavy late summer-fall use when the protein content and palatability is greater than maturing grasses and grass-like plants. Willows, and sedges decrease. As willows decrease, reed canarygrass becomes strongly dominant. In areas of low gradient, meandering, deep narrow streams with ready access to the floodplain, reed canarygrass holds overhanging banks together. In areas of straightened or higher gradients, lower successional states often occur with various degrees of bank erosion.
With loss of willows and lack of bank and toe cover, channels rapidly degrade during run-off events. Floodplain connectivity is lost, flows become concentrated, velocities increase and erosion is accelerates. The primary channel becomes deeper and wider in essence becoming a transportation reach. As the water table drops subsurface flows and storage of water for late season flows are reduced, the site becomes drier and production decreases. The initial rapid down cutting is followed by continued degradation as the incised channel widens. With widening of the incised channel an entrenched narrow flood plain slowly develops.
On lower elevation bottomlands channel straightening, deepening and drainage practices are often implemented to use the excellent floodplain soils for intense agriculture activities, transportation corridors and urban development. The hydrology effect is the conversion of a deposition reach with an active floodplain to a sediment transportation reach. With a narrowed steeper reach, stabilization practices are needed in combination with natural processes to promote the development of an entrenched stable narrow floodplain.
State 1
Historical Reference State
This represents the historical Reference State in pristine conditions. This State may be uncommon due to widespread invasion of reed canarygrass into this site. Erosion and deposition processes are within a historical range of variation, variability in depth to water table and seasonal fluctuations support native vegetation and vegetated communities include all historical functional and structural groups. The historical disturbance regime is intact and driven primarily by climate which influences drought and flood cycles. The resilience and resistance of the site is bolstered by negative feedbacks between vegetation establishment and hydrologic processes that maintains a dynamic equilibrium with geomorphological processes.
Dominant plant species
-
narrowleaf willow (Salix exigua), shrub
State 2
Current Potential State
SAEX/PHAR3 - Coyote Willow/Reed Canarygrass
This State is similar to the reference state with the exception of invasion of reed canary grass into the site. Erosion and deposition processes are still within a historical range of variation, yet are at risk of transitioning to a less stable state. All structural and functional plant groups are still present, yet composition and richness may be reduced. The historical disturbance regime is intact and driven primarily by climate which influences drought and flood cycles. The resilience and resistance of the site is bolstered by negative feedbacks between vegetation establishment and hydrologic processes that maintains a dynamic equilibrium with geomorphological processes. This state is common due to widespread invasion of reed canary grass in the Western US.
Community 2.1
Coyote Willow/Reed Canarygrass
Inside curves with point bars typically make up 20 percent of this site's mapping unit. Silty point bars are initially dominated by sedges and bulrush while narrowleaf willow, a rhizomatous species, occurs on gravelly bars.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Tree |
1295 |
1945 |
2589 |
| Grass/Grasslike |
891 |
1334 |
1782 |
| Shrub/Vine |
56 |
84 |
112 |
| Forb |
– |
– |
– |
| Total |
2242 |
3363 |
4483 |
State 3
Moderately Stable Banks, Connected Floodplain
(PHAR3/ALPR3) Reed canarygrass/Meadow foxtail
This State is characterized by a low gradient, meandering, deep and narrow stream with ready access to the floodplain. Chanel types are E and C, gradient is less than 0.2/100 ft, width to depth ratio is less than 12 and banks are moderately stable. Composition of willow and other native woody riparian vegetation has been reduced significantly, reducing mid summer shade to less than one percent and making the site vulnerable to channel widening and incision during large runoff events and transitioning to State 4. Reed canarygrass may provide bank stability of overhanging banks, yet it declines where connectivity to the water table is not available. Point bars are characterized by sand, silt and gravels supporting sedge, bulrush and narrowleaf willow. This state also includes stable analogue channels that have reformed following channel incision, widening and creating new floodplains.
State 4
Unstable Banks, Disconnected Floodplain
PHAR3 - Reed canarygrass
This State is characterized by straightened reaches with higher gradients, reduced sinuosity, unstable banks and disconnected floodplains. Chanel types are C, sinuosity is less than 1.5, gradient is greater than 0.2/100 ft and width to depth ratio is variable. Reed canarygrass has become strongly dominant where willows and other woody riparian vegetation have been reduced. This leads to bank erosion and reduced stream shading (less than one percent in mid summer). Channel widening and incision are common in this state as unstable banks and vegetation loss create a positive feedback loop that decreases resilience to runoff events. Point bars may still support narrowleaf willow and are characterized by gravels, as sand and silt has been lost. Abandoned floodplains transition into terraces and are dominated by drought adapted species that do not require a connection to the water table.
Transition T1A
State 1 to 2
Invasion of reed canarygrass
Transition T2A
State 2 to 3
Improperly managed grazing during times of year when willow is most vulnerable to decline or most susceptible to overuse.
Transition T2B
State 2 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests and significant alterations of upland watershed vegetation altering peak discharge or sediment loads.
Restoration pathway R3A
State 3 to 2
Restoration of hydrologic and biotic process and function through rehabilitation of channel and vegetation structure may be possible but will require considerable inputs, time and cost. This may require the placement of large woody debris, creation or removal of impoundments, alteration of water withdrawals, management changes to adjacent agricultural or grazing practices, or mechanical manipulation of stream channel courses among other intensive interventions. Restoration options will be highly site specific and may not be possible in many circumstances.
Transition T3A
State 3 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, destabilize banks, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests; sustained improperly managed grazing for many seasons; and significant alterations of upland watershed vegetation altering peak discharge or sediment loads. This state will be more vulnerable to these changes compared to State 2 given less stable banks and lower cover of riparian woody vegetation.
Restoration pathway R4A
State 4 to 2
Restoration of hydrologic and biotic process and function through rehabilitation of channel and vegetation structure may be possible but will require considerable inputs, time and cost. This may require the placement of large woody debris, creation or removal of impoundments, alteration of water withdrawals, management changes to adjacent agricultural or grazing practices, or mechanical manipulation of stream channel courses among other intensive interventions. Restoration options will be highly site specific and may not be possible in many circumstances.
Transition T4A
State 4 to 3
Given time, if natural channel evolution processes are allowed to take place, and sediment loads are sufficient, the stream will form an entrenched floodplain at a lower depth then the original. The original floodplain will become a terrace, disconnected from the water table and supporting drought adapted plant species. The resulting riparian area will be more confined and of significantly less extent than originally and the capacity of the basin to store water will be reduced considerably.
