Riparian

Description

This represents the most late-seral state observed during ESD development, and does not necessarily reflect all Pre-Columbian plant communities. It assumes a dynamic system with regular periods of flooding associated with sediment transport and deposition that result in channel migration within its meandering active floodplain. The presence or absence of beaver plays a role in retention of sediment and thus retention or storage of water in the landscape that modulates peak flow and helps to maintain a more consistent baseflow.

Active Floodplain (AF) - The active floodplain has the typical components of a meandering river channel; the point bar and cut-banks. These are dynamic landforms that may change position over time as the channel migrates naturally. The channel regularly accesses the active floodplain as water levels fluctuate with normal stream stage. These sediments are usually within a meter or less to the water table and seasonally are inundated.
The substrate associated with these features will consist of recent deposited stratified materials that form soils such as Ustifluvents or Haplustolls. Hydric soil components are present as wetlands in these landforms where old cutoff channels form swales in the cutbank floodplain, or where water stagnates along the point bar. The latter condition is more common along downstream portions of the point bar, backwater swales, or where materials are less sandy and gravelly. Hydric soils make up 15 to 45 percent of this component.
Stream channels are of the C and E types which are described as highly sinuous and meandering, low gradient, stable, lower width to depth ratios and with broader connected floodplains.

Floodplain Step (FS) - The higher position floodplain step will still receive overbank flooding on a regular basis, but may only occur during extreme events, perhaps 10-year or 20-year cycles. These positions are characterized by loamier materials that exist in lower energy overbank waters and rarely have fragments within a meter of the soil surface. In the Reference State, flooding will occur seasonally or every few years during high stream stages resulting in erosion or aggradation of their margins during these events. The channel water has regular access to the floodplain during flood events which acts to disperse the energy across a greater surface area.
Diversity of plant species and density of foliar and basal cover encourages the cycling of organic matter and therefore nutrients. Infiltration is high, allowing more moisture to be retained in the landscape, and which encourages resilience to drought periods. Good canopy cover shades the ground, keeping it cooler, and armored from rain-splash erosion and during periods of flooding. Rates of organic matter cycling and storage are in long-term equilibrium and only fluctuate slightly with annual variation in climate and major flooding events. Fire is a natural cycle that also encourages recycling of nutrients and maintains a grass and forb advantage over shrub encroachment.

Active Channel (AC) - The active channel course is not characterized by a soil profile, but is usually composed of interbedded gravels and sands with some areas of siltier deposits. During high flow stages, there may be some inundation of the point bar which results in a temporary subaqueous soil with emergent wetland vegetation. Channel stream shapes are best characterized by the Rosgen cross-section configurations C and E. Entrenchment ratios are relatively high, meaning the width of the floodplain accessed by an overbank event is at least twice the width of the channel at bank-full conditions. In addition, higher sinuosity and lower stream gradient are characteristic of these stream channel types.

Submodel

Description

Channel has been confined, straightened or watershed hydrology altered in terms of flooding frequency so that channel becomes incised or entrenched. This is a result of diverted waters for irrigation purposes or installation of flood control and water storage structures upstream that negatively impact seasonal baseflow and flooding events. As a result, the water table drops and bank storage decreases.
Some other scattered encroachment or connection to road crossings, diversion ditches, or other structures along and into the riparian zone. It is likely that the stream terraces connected to these riparian zones have been utilized for crop production which involves plowing, irrigation, drainage systems, as well as some access roads. Grazing of livestock is common within and along the corridor, without seasonal deferral or grazing plan designed to benefit riparian ecology. This may result in decreased flow, lower sinuosity, increased slope or stream velocity, and typically a lower entrenchment ratio.
These hydrologic shifts result in a deterioration of environmental services such as: lowered baseflow of stream during dry periods, increased severity of drought periods, increased erosion of stream banks and associated formation of lateral gullies, and loss of wetland acreage.
Loss of wetlands reduces ecological services such as: storage of organic matter in the soils which provide increased water holding capacity and nutrient retention; scrubbing pollutants from upland waters before reaching the stream through denitrification of organic N (reduced to gaseous N compounds such as N2); providing habitat for rare hydrophytic plants that tend to be good forage for wildlife and livestock and habitat for rare and endangered species.
The change in water table height may cause a shift or reduction in riparian habitat along the banks, plants such as willows may struggle to exist. In these situations, the banks may become un-vegetated or under-vegetated in some areas and susceptible to erosion during flood events. This destabilization of channel banks will release sediments that will not only increase turbidity in the water, and silt up downstream water storage structures, but it will cause a change in the channel/floodplain dynamics such as width to depth ratios or entrenchment ratios.

Active Floodplain (AF) - Often a decrease of willow species occurs with an increase in blue grama, smooth brome, inland saltgrass and annuals such as bindweed, lambsquarters, and kochia. Encroachment of exotic species such as salt cedar and Russian olive occurs, watch for presence of cocklebur as a sign of further degradation. Associated resources are compromised relative to the reference state which include a decrease in functioning wetland acreage and a decrease in infiltration and storage of groundwater. The hydric soils are only a fraction of their historic size, ranging from 5 to 20 percent of the component. Hydric soils are constrained as smaller narrow wetlands along margins of point bars, depressions from old oxbow cutoffs, or very narrow micro floodplains along entrenched channel while stream stage is low.
Loss of tree cover due to removal of bosque for wood products, or to increase understory production. It is also possible that the tree community has disappeared over time due to lowering of water table from altered flow and diversions upstream. Recruiting and regeneration has been unsuccessful over time, resulting in total loss of bosque ecosystem.

Floodplain Step (FS) – Optimized for access to pasture grazing, often roads or other disturbances or cropland are along edge or within this area.

Active Channel (AC) – Associated with managed-altered riparian system, the channel will respond with a greater width to depth ratio due to erosion of the banks and deposition of sediment in the channel. This, in addition to loss of bosque structure and willows along the banks will increase the temperature of the stream water which adds stress to some native aquatic populations.

Submodel

Description

Active Floodplains (AF) – Highly impacted by engineered channeling, near transportation corridors, highly diverted water from channel into irrigation structures characterize the geomorphic changes to this riparian zone. Bank-full flood stages rarely access this floodplain due to significant entrenchment of channel. Some widely scattered cottonwoods and willows but significant exotic woody species will have encroached in wetter areas, and mostly shrubs and upland grasses or forbs in drier areas.
Formerly incised channels have eroded the banks and increased the width to depth ratio in return. A new miniature floodplain system may have developed in this widened bottom, a small fraction of the former riparian corridor.
The water table may still be seasonally accessible due to discharge of upland landscape moisture during normal or wetter years, but can be absent on drier years. This will prevent willows from growing here, but not other upland shrubs such as fourwing saltbush and Apache plume.

Floodplain Step (FS) –Characterized by upland plant communities due to history of plowing for crops or for pasture grazing, with an increase in bare ground and annuals. Bank-full flood stages never access this floodplain, it’s essentially abandoned and acts as a stream terrace, absent a restoration plan to reconnect the hydrology. Arguably the historic active floodplain is now the floodplain step, but is limited in acreage in comparison.
Soils will likely be excessively drained since they commonly have subsurface horizons that are a minimum sandy but usually gravelly or cobbly textures. They act to further drain the upland hydrology more efficiently than historic catena functions. Storage of organic matter in the soils has declined due to erosion and/or the change from a high species diversity with canopy of 80 to 100 percent cover to a low diversity with a canopy ranging from 50 to 100 percent cover. Production has declined due to less consistent access to soil moisture from a seasonally fluctuating water table to a rarely accessible water table.

Active Channel (AC) – Entrenched or seasonally dry due to diverted stream waters. Stream channel type G with low entrenchment ratios and width to depth ratios, increased stream slope and bank erosion rates.

Submodel