This riparian system is found throughout the Rocky Mountain region within a broad elevation range from approximately 1,525 to 2,750 m (5,000 to 9,000 ft). In Colorado, it is primarily found in the western half of the state and along the foothills of the eastern mountain front. Seasonal and episodic flooding is the primary driver of this ecosystem and is essential to maintaining a mosaic of plant associations. Vegetation in this system is characterized by a mosaic of multiple communities that are tree-dominated with a diverse shrub component. Component plant associations vary with elevation, stream gradient, floodplain width, and flooding events. Dominant trees may include box elder (Acer negundo), narrowleaf cottonwood (Populus angustifolia), plains cottonwood (P. deltoides ssp. monilifera), Rio Grande cottonwood (P. deltoides ssp. wislizeni), lanceleaf cottonwood (P. x acuminata), balsam poplar (P. balsamifera), Douglas fir (Pseudotsuga menziesii), blue spruce (Picea pungens), peachleaf willow (Salix amygdaloides), or Rocky Mountain juniper (Juniperus scopulorum). Dominant shrubs include Rocky Mountain maple (Acer glabrum), thinleaf alder (Alnus incana), river birch (Betula occidentalis), red-osier dogwood (Cornus sericea), river hawthorn (Crataegus rivularis), stretchberry (Forestiera pubescens), chokecherry (Prunus virginiana), skunkbush (Rhus trilobata), mountain willow (Salix monticola), Drummond's willow (S. drummondiana), narrowleaf willow (S. exigua), dewystem willow (S. irrorata), Pacific willow (S. lucida), buffaloberry (Shepherdia argentea), or snowberry (Symphoricarpos spp.). Exotic trees such as Russian olive (Elaeagnus angustifolia), crack willow (Salix fragilis), and tamarisk (Tamarix spp.) are common in some stands.

This riparian ecosystem is differentiated from other riparian systems by elevation, vegetation and hydrology. Hydrology in this system is primarily snowmelt driven with annual and episodic flooding. Vegetation is generally characterized by cottonwood (Populus spp.) woodlands, though other tree species can dominate. Common shrub species include willow (Salix spp.), thinleaf alder (Alnus incana), hawthorn (Crataegus spp.), red-osier dogwood (Cornus sericea), and river birch (Betula occidentalis).

This system is found throughout the Rocky Mountain and Colorado Plateau regions from New Mexico and Arizona north to Idaho and the island mountain ranges of central and eastern Montana. In Colorado this system is found primarily in the western half of the state within a broad elevation range from approximately 1,525 to 2,750 m (5,000­­-9,000 ft). A zone supporting instances of this system is also found along the foothills of the eastern mountain front.

This is a linear system confined to narrow bands that generally parallel the linear and branching configuration of stream channels and lakes.

Riparian systems are dynamic and in an interdependent state of flux as they are intimately connected with surface or subsurface hydrologic systems and stream ecosystems. This ecological system is a linear continuum that is found within the flood zone of rivers, on islands, sand or cobble bars, and immediate streambanks. It can form large, wide occurrences on mid-channel islands in larger rivers or narrow bands on small, rocky canyon tributaries and well-drained benches. It is also typically found in backwater channels and other perennially wet but less scoured sites, such as floodplains swales and irrigation ditches. It may also occur in upland areas of mesic swales and hillslopes below seeps and springs. These communities are also typical of canyon streams, which have generally steeper gradients and coarser soils than plains rivers or streams.
Climate in the range of this system is continental, with historically cold winters and hot summers and an overall semi-arid climate. Depending on elevation, the average annual temperatures vary from approximately 40^o to 60^o F while average precipitation ranges from approximately 6 to 10 inches at the base of the mountains to 40 inches at higher elevations. Most precipitation occurs as snowfall during the winter months (as much as 80% at high elevations) and thus is the most important source of water for wetlands in the Southern Rocky Mountains, although late-summer convective thunderstorms produce slight peaks in runoff in late summer.
Water for this system can originate as overland flow, shallow subsurface storm flow, direct precipitation, and recharge from local alluvial aquifers. Surface water flow and flooding is a function of snowmelt, watershed and valley topography and area, late-summer rainfall, and the extent of upstream riparian wetlands. Flooding from the stream channel recharges many alluvial aquifers and as stream flow decreases the trend is reversed as the alluvial aquifer begins to recharge stream flow.
Soils are typically alluvial deposits of sand, clays, silts and cobbles that are highly stratified with depth due to flood scour and deposition. Highly stratified profiles consist of alternating layers of clay loam and organic material with coarser sand or thin layers of sandy loam over very coarse alluvium. Soils are fine-textured with organic material over coarser alluvium. Some soils are more developed due to a slightly more stable environment and greater input of organic matter.

This system consists of temporarily, seasonally and intermittently flooded woodlands and shrublands dominated by broad-leaved deciduous species, both in the tree and shrub canopy, as well as occasional conifers. At montane elevations (up to 2,750 m or 9,000 ft), narrowleaf cottonwood (Populus angustifolia) is the characteristic tree species and may intermix or give way to conifers such as Douglas fir (Pseudotsuga menziesii) on north facing slopes of canyon floors or to blue spruce (Picea pungens) adjacent to streams. At foothill elevations (below 2,130 m or 7,000 ft), tree species such as Rio Grande cottonwood (Populus deltoides ssp. wislizeni) are typical of the west slope, while plains cottonwood (P. deltoides ssp. monilifera) occurs on the east slope. Cottonwoods often occur with the shrub coyote willow (Salix exigua), common on both slopes. At both montane and foothill elevations, the tall shrubs thinleaf alder (Alnus incana), river birch (Betula occidentalis), and red-osier dogwood (Cornus sericea) are common understory dominants. Although these three species are often found together or alternating along a reach, there is a slight tendency for alder to be more common at higher elevations and dogwood at lower. Other woody species that may be present include skunkbrush (Rhus trilobata), rockspirea (Holodiscus dumosus), roundleaf snowberry (Symphoricarpos rotundifolius), western snowberry (Symphoricarpos occidentalis), box elder (Acer negundo), golden currant (Ribes aureum), buffaloberry (Shepherdia argentea), poison ivy (Toxicodendron radicans), Wood's rose (Rosa woodsii), stretchberry (Forestiera pubescens) and a variety of tall willows (Salix spp.).
The herbaceous layer is relatively sparse and is typically graminoid dominated. Wet meadow or emergent marsh community types may occur in a mosaic with this system. Associated species may include mountain rush (Juncus balticus var. montanus), common spikerush (Eleocharis palustris), saltgrass (Distichlis spicata), wildrye (Elymus spp.), horsetail (Equisetum spp.), foxtail barley (Hordeum jubatum), bulrush (Schoenoplectus spp.), scratchgrass (Muhlenbergia asperifolia), western wheatgrass (Pascopyrum smithii), giant reed (Phragmites australis), Canada goldenrod (Solidago canadensis), starry false lily of the valley (Maianthemum stellatum), sedges (Carex spp.), and various non-native graminoids. The understory may contain substantial coverage of bare soil, gravel, cobbles, and boulders.

Lower elevation riparian woodlands and shrublands in this system have some of the richest avian species component of any of Colorado's habitats. Characteristic bird species here include American Kestrel (Falco sparverius), Western Screech Owl (Megascops kennicottii), Great Horned Owl (Bubo virginianus), Mourning Dove (Zenaida macroura), Northern Flicker (Colaptes auratus), Western Wood-Pewee (Contopus sordidulus), Western Kingbird (Tyrannus verticalis), Eastern Kingbird (Tyrannus tyrannus), House Wren (Troglodytes aedon), Black-billed Magpie (Pica hudsonia), American Robin (Turdus migratorius), Blue Grosbeak (Passerina caerulea), and Bullock's Oriole (Icterus bullockii). Characteristic species in foothills and lower montane riparian deciduous systems include Yellow Warbler (Setophaga petechia), Warbling Vireo (Vireo gilvus), Song sparrow (Melospiza melodia), and Broad-tailed Hummingbird (Selasphorus platycercus). In riparian coniferous communities in these elevations, commonly detected bird species include Cordilleran Flycatcher (Empidonax occidentalis), Ruby-crowned Kinglet (Regulus calendula), Golden-crowned Kinglet (Regulus satrapa), Swainson's Thrush (Catharus ustulatus), Mountain Chickadee (Poecile gambeli), Yellow-rumped Warbler (Setophaga coronata), and Western Tanager (Piranga ludoviciana). Birds identified as high priority species include Lewis's Woodpecker (Melanerpes lewis), Lazuli Bunting (Passerina amoena), and Western Kingbird (Tyrannus verticalis). The federally listed as threatened Preble's meadow jumping mouse (Zapus hudonius preblei) is found exclusively in foothill riparian areas of the eastern mountain front. Smooth green snake (Opheodrys vernalis), boreal chorus frog (Pseudacris maculata), and barred tiger salamander (Ambystoma mavortium) may also be found in this habitat.

Vegetation in this riparian ecosystem is distinct from uplands because of the greater availability of water in an otherwise water-limited landscape. In a natural condition, this system often occurs as a mosaic of multiple, tree-dominated communities with a diverse shrub component and a complete range of age-classes. Key variables driving vegetation dynamics in riparian sites include base and mean flow levels, the magnitude, seasonality, and frequency of peak flows, and the relative influence of groundwater on water tables. Episodic flood events and channel migration cause pulses of woody species establishment creating patches of different age classes. In areas where the water table drops below the stream channel, species composition is characterized by deep rooted phreatophytes like cottonwoods or upland species tolerant of low soil moisture.
Riparian ecosystems are, in general, maintained by an active natural disturbance regime of which floods are the most important disturbance type. Natural stream flow regimes in Rocky Mountain riparian systems are distinctive, with a strong annual peak discharge in spring driven by snowmelt and periodic large floods or debris flows from storm events that have the potential to move tremendous amounts of sediment. These interactions between hydrologic and geomorphic processes result in overbank flows, deposition, and lateral migration, creating a mosaic of landforms such as channels, floodplains, point bars and in-channel islands, which drive the spatial pattern and successional development of riparian vegetation. Riparian woodlands and shrublands thus grow within a continually changing, dynamic alluvial environment due to the ebb and flow of the river, and riparian vegetation is constantly being "re-set” by flooding disturbance.
Riparian zones respond to the process of flooding disturbance and sediment deposition by establishing successional stages of plant growth on the newly deposited sand and gravel bars. Thus these riparian ecological systems, as exemplified by cottonwood communities, contain early-, mid- and late-seral riparian plant associations as well as non-obligate riparian species. Cottonwood (Populus spp.) often forms extensive woodlands along streambanks of this system. Regeneration is dependent on flooding disturbance that occurs with appropriate timing and frequency and with sufficient magnitude and duration to deposit wet, bare alluvium in full sun to enable germination and establishment. Thus mature cottonwood occurrences do not regenerate in place, but by "moving" up and down a river reach with the meandering stream. Over time a healthy riparian area supports all seral stages of cottonwood communities.

Primary threats to this system include hydrologic alteration, riparian and upland vegetation alteration and development, and removal of beaver. The primary abiotic ecological process necessary to maintain this ecological system is hydrology and more specifically surface flow. Global climate change is resulting in snowpack declines and earlier snowmelt due to unprecedented springtime warming foreshadowing fundamental impacts on streamflow across the western United States. Regional climate characteristics of particular importance for riparian ecosystems include the amount, form, and seasonality of precipitation, temperature and wind, and the timing of snow-melt. These factors influence stream flow regimes, including flow variability and characteristics such as base, mean, and peak flow volumes, and fluvial geomorphic processes such as sediment erosion and deposition.
Riparian woodlands are dependent on specific, natural hydrologic regimes, soils, as well as the physical space that provides the ability to move both up and down the stream as well as side to side within the floodplain. Development such as roads, that constrain the ability of the stream to migrate, or alteration of the natural flooding regime by development activities such as water impoundments and diversions may produce changes in plant composition and community structure and thus stream and riparian function. For instance, reduced magnitude and frequency of floods has allowed the invasion of invasion of drought tolerant exotics such as tamarisk (Tamarix spp.) and flood intolerant species such as rabbitbrush, and hastened the decline of mature cottonwood trees on floodplains.
Riparian ecosystems are functionally connected to upstream and downstream ecosystems and are laterally connected to upland and aquatic ecosystems. Alteration to upland landscapes by urban or agricultural uses often disconnects upland from riparian and aquatic habitat. Alteration of the riparian zone by activities, such as livestock grazing or urbanization, that disturb riparian community structure and/or species composition may destabilize streambanks, resulting in excessive bank erosion and downcutting thereby disconnecting the stream from its' floodplain and riparian habitat.
Historically, one of the most important modifiers of stream/riparian ecosystems throughout the Rocky Mountains was the North American beaver - their current ecological importance derives from the consequences of their extirpation by trapping. Beaver exert a strong influence on both stream and riparian habitat by building that dams that slow flooding flows, trap sediment, allow water to spread out into adjacent habitat and gradually fill in to create marsh or wet meadow systems. Thus the stepped profiles of beaver-influenced rivers maximize the diversity of riparian and aquatic habitats. The effect of the removal of beaver along rivers is a reduction in habitat diversity and stability as channels incise, snowmelt flood peaks increase, flood-related sediment transport increases and riparian and slow-velocity habitats are lost.