Ecological dynamics

The natural gorges within MLRA 35 have low gradient B4c channels that have developed flood plains that are able to support cottonwoods. The streams within this site are perennial with regular spring runoff high flow and intermittent flash flooding during summer monsoon season. Instream large wood may also contribute to channel form, allowing sediment to accumulate and build banks. This process would occur when mature cottonwoods fall in the channel, or when large wood from upstream riparian trees are deposited on the bank after flooding. Beaver activity could also provide input of large wood, although no recent beaver activity was found on any riparian complexes within this ecological site.

Large magnitude floods may occur July through October within this MLRA. These floods are generally the result of heavy rain in the summer or fall. Channel morphology can remain stable with large floods, although vegetation can be scoured as a result. However, vegetation found in this type of flashy system is adapted to this type of disturbance. Coyote willow has very flexible stems that bend with the water and the dominant herbaceous vegetation (Baltic rush, common threesquare, and common reed) grows up through fresh sediment deposits (McBride & Strahan 1984; Anderson 2006).

Historic land management practices may have influenced current channel morphology directly or indirectly through upland degradation, but the effects of the management practices have not been adequately documented to establish a cause and effect relationship within this site. Over use by livestock would have a similar effect in the uplands and if overuse occurred in the canyon bottoms, the removal of bank stabilizing vegetation could increase bank failure, accelerating bank erosion and undermining channel stability. Perennial water in this region is a rare and valuable resource and is often exploited for human use. Perennial channels in this region have commonly been altered by irrigation withdrawals, impoundments, and channelization. These uses can influence the channel geomorphology and vegetation structure of this ecological site. Altered hydrology can affect the establishment of cottonwoods because cottonwood germination is dependent on timing and magnitude of flooding (see next section). Channel geometry is also determined by hydrology. Bankfull events (1.5 to 2 year flood events) are what shapes the channel, if those channel forming flows are altered, the channel geometry will also change and adjust to the new hydrology.

The state and transition model is based on a customized (derived from documented scenarios) stream succession scenario of B4c > G4/F4. This may be due to bedrock controlling the stream grade; instead of the stream channel down cutting, the stream moves within the flood plain, creating a new channel.

General: This riparian complex is a low gradient (12) width/depth ratio. The site extends throughout MLRA 35 including streams that have the same potential channel morphology, channel succession, bed and bank materials, fluvial surfaces, potential plant community components, and similar response to disturbance.

Plant Communities and Fluvial Surfaces: This system exhibits a complex of narrow streambanks, flood plains and flood-plain steps. The streambanks and flood plains are composed of alluvial sediment that is generally sandy. Vegetation composition is influenced by flood frequency, flow duration and length of inundation. Fluvial surfaces that are closer to the channel are inundated more frequently with floods, thus have less woody vegetation. The vegetation growing on the fluvial surfaces close to the channel have more access to ground water, allowing more obligate wetland species to dominate. Fluvial surfaces further from the channel are inundated less frequently and have greater woody vegetation cover and also have less access to ground water except through deep roots. Dominant vegetation in this ecological site is adapted to yearly variations in flow and sediment deposition, cottonwoods and willows particular to this site are adapted to frequent disturbance and are known to be aggressive colonizers of disturbed sites (Richenbacher 1984). The adaptations of willows and cottonwoods are: they produce a large number of seeds, the seedlings have a high growth rate, stem fragments can regenerate, and willow root systems are extensive and allow the plant to anchor and bind the soil (Karrenberg et al. 2002). Willows and cottonwoods also require fresh wet sediment that is devoid of other vegetation to germinate (Braatne et al. 1996). Yearly variations in flow and large floods that scour vegetation and deposit sediment on floodplains are ideal microsites for willow and cottonwood seeds. Cottonwoods and willows produce large amounts of wind and water dispersed seeds that are only viable for a short period of time after the seeds come in contact with moist soil (Braatne et al. 1996). Germination can occur quickly, usually in a 24 hour period (Karrenberg et al. 2002) and they will remain viable for 2 to 3 days (Braatne et al. 1996). Mortality of cottonwood and willow seedlings is very high ranging from 77 to 100% in the first year (Karrenberg et al. 2002). Mortality is often attributed to desiccation of the seedling and seedlings are at a great risk of subsequent summer floods that may scour the recently deposited sediment (Braatne et al. 1996; Karrenberg et al. 2002).

The stream banks/flood plains are very narrow and are correlated to plant community component 1 (PCC1). They are close enough to the water table and water in the channel to support obligate wetland vegetation. Plant communities that are found closest to the channel on the stream banks are rushes, common reed and mesic forbs. These rhizomatous plants have the ability to survive frequent high flow and can send stems up through freshly deposited sediment. Willows and mesic forbs often establish on the upper edge of the narrow flood plain away from the channel because it is an intermediate location between high flows and access to the water table. Coyote willows found in this community have flexible stems and are able to bend with high flow (Karrenberg et al. 2002; McBride & Strahan 1984). Native fish and other aquatic species, including frogs and aquatic insects, are present, utilizing floodplain wetlands for refuge during high flows, and deeper residual pools as drying occurs during the drier months of the year.

Flood-plain steps are not flooded as frequently and are dominated by a mix of facultative wet and upland vegetation (PCC2). The mesic species in these communities probably established when the water table was closer to the soil surface or established after a large flood in a wet year and the upland species establish between wet years or high flow events. The flood-plain steps support Fremont cottonwood generally with upland species in the understory such as basin big sagebrush, rabbitbrush, and Indian rice grass. Cottonwood survival is based on the proximity to water in the channel and in the water table. The establishment of cottonwoods and coyote willow is cyclical, typically after a flood. These events deposit the needed wet sediment for germination. The likelihood of cottonwoods reaching maturity decreases if they germinate closer to the channel, because of the susceptibility of the streambank to high flow and scouring.

Invasive Species: Tamarisk, Russian olive and Russian thistle are often found within this ecological site. Tamarisk is the most common invader and can readily replace the willows and cottonwoods. If the channel abandons floodplains and terraces, the groundwater influence decreases which can create a better environment for tamarisk to establish (Horton et al. 2001). Tamarisk is more tolerant of drought and salinity than native species (Horton et al. 2001). The timing of seed dispersal is also different for tamarisk than native shrubs. Tamarisk produces seed from April to October (Horton et al. 2001) and with high summer flows could be at a seed dispersal advantage over cottonwoods and willows, which produce seed from February to April (Braatne et al. 1996). Tamarisk seedlings can establish midsummer on fresh sediment deposits from runoff during summer rain storms, months after cottonwood and willows dispersed seed (Stromberg et al. 2007). Tamarisk invasion can create a feedback loop that is difficult to reverse. Once tamarisk invades and begins to replace willows and cottonwoods, bare soil begins to decrease and shade increases, further decreasing the chance that cottonwoods and willows will regenerate in the site.

Fire: Fire has not been found to be a major factor contributing to plant community change. The landscape is dissected by deep canyons and the uplands do not support high grass production that would allow a fire to spread. Fremont cottonwood is not as tolerant of fire as narrowleaf cottonwood, indicating that the dominant plant community is not adapted to frequent fire. Fremont cottonwood can be top killed even by a moderate fire and the cambium layer has been found to be damaged by a low severity fire, although it can sprout from the bole and roots (Taylor 2000).

Plant Community Components
These plant communities exist on specific fluvial surfaces (PCC1 on stream banks/flood plains and PCC2 on flood plain step). These communities may exist over the entire length of the site and vary slightly to moderately in plant community composition.

Plant Community Component 1
This community is frequently disturbed by flash flooding. Shrubs and trees are generally not found within this plant community because of the close proximity to the stream channel and scouring floods. Herbaceous plants can establish between flooding events and can often come up though sediment if they are buried during a flood event. Baltic rush is a sod forming, rhizomatous perennial graminoid that forms a dense root system that can bind non-cohesive stream bank sediment. Baltic rush can tolerate fluctuating hydrology. It is adapted to flooding and drought. It is also tolerant of a range of soil conditions, including mild to moderate salinity and alkaline to calcareous soils. Common threesquare is similar to Baltic rush, it is also rhizomatous and it tolerant of alkaline conditions. Coyote willow is often found on the upper edge within this community because it is able to expand through rhizomes and grow under conditions that it would not be able to germinate (Anderson 2006). Roots of graminoids and coyote willow hold the stream banks during typical flood events, but may be scoured during large floods. Willows have flexible branches that are able to bend with the force of water without much damage to the plant (Karrenberg et al. 2002; McBride & Stahan 1984). Coyote willow (Salix exigua) is drought resistant and very tolerant of flooding (Anderson 2006) an essential characteristic in the dry environment of southern Utah. Willow seeds are non-dormant and quickly loose viability. Seedlings generally establish close enough to a water supply and far enough from the channel to be protected from scouring during floods (Anderson 2006). Once established, vegetative clones can expand perpendicular to the stream channel, developing closer to the water source (Anderson 2006). Branches can resprout if buried by sediment and they may also regenerate vegetatively from broken stems and roots (Anderson 2006). Coyote willow is shade intolerant and is shaded out once cottonwoods grow tall enough to dominate the overstory (Karrenberg et al. 2002). Seedlings of willow and cottonwood require the same germination conditions, bare, moist soil so they often germinate together. Initially willows grow faster than cottonwood, but given time cottonwood overtops the willows. The reason cottonwood is not the dominant overstory plant in this plant community is because it is too close to the channel and is scoured before it can grow to maturity. Young cottonwood was found scattered on this fluvial surface but was not dominant.

This community initially develops on streambanks and may become established on exposed depositional sand bars. Establishment of this community depends on periodic flooding for maintenance and growth. As sediment and debris become trapped among woody stems, the bar becomes more stable. This community occurs on flood plains with sufficient sediment deposition. Downed wood is sparse.

Plant Community Component 2
Fremont cottonwood is the dominant overstory plant in this community. Narrowleaf cottonwood occurs within this community, but is not as prevalent as Fremont cottonwood. These two species are known to hybridize, creating the hybrid Populus xhinckleyana (Eckenwalder 1984). The vegetation is sparse within this plant community. It is dominated by a cottonwood overstory with various shrubs in the understory. Grasses and forbs are scattered, but not as dominant understory plants. Intermittent flooding maintains this flood-plain step. Due to the deep water table on the flood-plain step, understory species are not riparian obligates and only those species that are deep rooted, such as cottonwood, can access the water table.

Flooding generally reaches this terrace during high flows that occur in the summer monsoon season. Channels that are close to the canyon walls can receive large amounts of runoff from the surrounding uplands during flash flood conditions (see photo 6 under “State 1”). This occurrence may scour the flood-plain step more than spring flood events. Fluvial disturbance is essential in maintaining cottonwood communities. Fremont cottonwoods generally establish from seed and not asexual reproduction, and because of this they are generally found on channels that are affected by moderate flows (Rood et al. 2007). Seedlings and saplings have been observed in this plant community indicating that depositional events occur frequently. Fremont cottonwood flowers from February to early April allowing the seed to be dispersed late spring (May), optimally on the receding limb of the spring flood stage. Seeds established during this stage of flooding are not typically susceptible to subsequent flooding and the soil is generally moist enough to allow germination (Braatne et al. 1996, Rood et al. 2007). Germination may occur more often, but establishment and survival past seeding stage in to young stage is unlikely to occur every year. Cottonwoods generally only germinate on freshly deposited sediment and will generally not germinate in an area already covered by vegetation (Rood et al 2007). Fire is unlikely to be a driving force in this plant community because of the location in deep canyons and discontinuous vegetation along the channel. Fremont cottonwoods are not regenerated by fire and mature trees can be top-killed by moderate fires (Taylor 2000).