Western Great Plains Riparian
Click link below for details.
General Description
The Western Great Plains Riparian system is found in the riparian zones of along moderate-sized woody rivers, streams, and ephemeral drainages throughout the western Great Plains, including eastern Colorado's shortgrass prairie region. This system occurs on alluvial soils and is driven by periodic flooding and site capacity for tree establishment. Plains riparian systems can contain seasonally flowing open water channels or ephemeral and often sandy, sparsely vegetated washes. The landscape setting can be highly variable, ranging from incised canyon ravines to wide, braided streambeds. Hydrology of wash occurrences tends to be flashy and entire streambeds can be dry for some portion of the year. Primary inputs to this system are local precipitation events, overland flow, subsurface interflow from adjacent uplands, and groundwater discharge. Seasonal and episodic flooding is the primary driver of this ecosystem and is essential to maintaining a mosaic of plant associations. Communities within this system can intersperse along a single reach and include riparian forests and shrublands, tallgrass and mixed grass mesic meadows, herbaceous wetland swales and seep/springs, and gravel/sandbars. Some systems have a layered forest canopy, while others have scattered open trees, shrub patches, and herbaceous zones near the channel, outer floodplain, or short herbaceous reaches within a larger wooded landscape. Dominant species include plains cottonwood (Populus deltoides), willow (Salix spp.), silver sagebrush (Artemisia cana), western wheatgrass (Pascopyrum smithii), switchgrass (Panicum virgatum), vine mesquite (Panicum obtusum), sand dropseed (Sporobolus cryptandrus), foxtail barley (Critesion jubatum), blue grama (Chondrosum gracile), and saltgrass (Distichlis stricta). Localized marsh communities may occur within or adjacent to riparian zones, with plant species such as cattail (Typha spp.), bulrush (Schoenoplectus spp.), spikerush (Eleocharis spp.), and other sedges (Carex spp.). Hydrologic alteration, livestock grazing and/or agricultural activities are common anthropogenic impacts. Consequences of flow reductions and changes diminish riverine ecosystem function and lower water tables. This degrades altered vegetation composition and structure, and the invasion of non-native species including saltcedar (Tamarix spp.), Russian olive (Elaeagnus angustifolia), green ash (Fraxinus pennsylvanica) and nonnative grasses and forbs. Impacts can reduce site connectivity by channel downcutting, narrowing of wetland zones, vegetation infilling within the channel bank, and soil drying.
Diagnostic Characteristics
In Colorado, seasonal and episodic floods are key drivers of Western Great Plains riparian areas and create conditions for woody vegetation establishment. Localized flashy precipitation events regularly deposit sandy alluvium and wrack on vegetated riparian areas. Primary inputs to this system are groundwater discharge, local precipitation events, overland flow, and subsurface interflow from adjacent upland. In Colorado, Western Great Plains Riparian systems occur throughout the eastern plains occupying the riparian zone of 1st to 4th order streams, including ephemeral and intermittent streams. Vegetation can include a tree canopy dominated by plains cottonwood (Populus deltoides), a shrub layer dominated by willow, and/or an herbaceous layer with a variety of graminoids. Sites can contain a mix of wetland and upland species, depending on stream duration. Some reaches of plains riparian areas can resemble marshes, with pools of slow moving water dominated by coarse emergent vegetation.
Similar Systems
Rocky Mountain Lower Montane-Foothill Riparian Woodland and Shrubland: Rocky Mountain Lower Montane-Foothill Riparian Woodland and Shrubland systems occupy slightly higher elevations than Western Great Plains systems, ranging from 5,000-9,000 feet. Whereas plains riparian systems are primarily driven by local precipitation and groundwater inflow, foothill riparian zones are driven more by snowmelt in the mountains. Plains riparian systems are generally characterized by a tree canopy dominated by plains cottonwood (P. deltoides) and/or peachleaf willow (S. amygdaloides), and a shrub zone dominated by coyote willow (S. exigua); whereas foothills riparian systems tend to have higher shrub and tree cover, diversity, and vertical complexity. The transition between plains riparian and foothills riparian can be ambiguous where lower stream reaches that originate in the mountains extend far into the plains.
Western Great Plains Open Freshwater Depression Wetland: This headwater and small stream system is generally herbaceous and encompasses small streams before their processes transition from throughflow to overbank flooding. Colorados plains riparian system is intermediate in energy and watershed position between the wet meadow and marsh network and plains floodplain, where sites can support trees, but the floodplain is less developed. Riparian native herbaceous patches are more commonly mesic to upland grasses such as bluestem and needle-grasses, versus the dominant wetland graminoids in the wet meadow and marsh drainages. However, separate small patches of wetland herbaceous and marsh cover can exist in plains riparian seeps and swales, and prior to hydrologic alterations, woody understory composition likely had substantial components of native wetland species.
Range
This system is found in riparian areas of ephemeral to medium-sized streams and rivers throughout the western Great Plains. It is most common in Central Shortgrass Prairie and Southern Shortgrass Prairie ecoregions but extends west as far as the Rio Grande in New Mexico and into the Wyoming Basins. In Colorado, this system occurs on tributaries to the South Platte, Arkansas, Cimarron, and Republican rivers.
Ecological System Distribution
Spatial Pattern
Western Great Plains Riparian is a linear system.
Environment
Plains riparian systems are found within the floodplains of rivers, and on islands, sand or cobble bars, and immediate streambanks where they are intimately connected with surface or subsurface hydrologic processes. Water source for these streams systems varies by stream and even by reach, from flashy reaches driven by summer precipitation to perennial reaches connected to alluvial groundwater aquifers. Stream flow regimes are highly variable and are often water-limited for at least part of the year. Most plains streams are dominated by local rainfall events that typically produce low-volume, short-duration flows, and are generally dry or wadeable by mid-summer due to evapotranspiration and lack of precipitation.
The surrounding landscape of the western Great Plains has generally low topographic relief with level and gently rolling hills, but is occasionally punctuated by deep canyons and dramatic escarpments, buttes, mesas and volcanic peaks. Numerous streams and rivers dissect the landscape, flowing north, south, and eastward into the major rivers of the region. The Colorado plains are situated in the rain shadow of the Rockies, causing dry, warm and sunny summers with daytime temperatures often 95o F or above. Rainfall is below twenty inches per year, with 70-80% falling in the spring and summer during the growing season. Wind speeds are high, and their drying effects, together with high temperatures, cause soil drying and summer drought. Climate in riparian zones is similar to the surrounding upland habitat, however the increased cover from the tree canopy moderates intense winds and sunlight and provides protected habitat to support diverse flora and fauna.
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. Some soils are more developed due to a slightly more stable environment and greater input of organic matter.
Vegetation
Western Great Plains Riparian systems are characterized by narrow to wide stands of riparian forests or woodlands dominated by plains cottonwood, alternating sparsely wooded and grassland areas with shrublands dominated by willows, or open wooded washy gravel/sand flats. Riparian forest and woodland communities of this system are often dominated by plains cottonwood (Populus deltoides ssp. monilifera) but may include the hybrid lanceleaf cottonwood (Populus x acuminata), peachleaf willow (Salix amygdaloides), and occasionally oneseed juniper (Juniperus monosperma) in canyon systems. Other deciduous trees such as box elder (Acer negundo), green ash (Fraxinus pennsylvanica) may contribute to the canopy. Willow species (commonly Salix exigua) may form a conspicuous layer with cottonwood saplings near the stream channel, or may form the overstory layer. The understory composition and structure are variable. A shrub layer may be present, with species such as willow (Salix spp.), western snowberry (Symphoricarpos occidentalis), silver sagebrush (Artemisia cana), rubber rabbitbrush (Chrysothamnus nauseosus), and Woods' rose (Rosa woodsii).
The herbaceous stratum is variable. Drier sites contain graminoids such as blue grama (Bouteloua gracilis), foxtail barley (Horedeum jubatum), saltgrass (Distichlis spicata), Canada wildrye (Elymus canadensis), western wheatgrass (Pascopyrum smithii), and sand dropseed (Sporobolus cryptandrus). Wetter sites with a groundwater connection may contain Nebraska sedge (Carex nebrascensis), pale spikerush (Eleocharis macrostachya), common threesquare (Schoenoplectus pungens), or hardstem bulrush (Schoenoplectus lacustris). Subirrigated sites may support tallgrass meadows dominated by switchgrass (Panicum virgatum) or prairie cordgrass (Spartina pectinata). Common forbs species include poison ivy (Toxicodendron rydbergii), field horsetail (Equisetum arvense), showy milkweed (Asclepias speciosa), Canadian horseweed (Conyza canadensis), Canada goldenrod (Solidago canadensis), and American licorice (Glycyrrhiza lepidota).
Historically, most Western Great Plains streams and the surrounding grasslands appeared much differently than they do today. These historical riparian systems were a complex mosaic of plant communities, which served as an oasis for wildlife that provided shelter, nesting and foraging resources. Today the grasslands have largely been replaced by agricultural fields and open range, some rivers have been narrowed into single-thread channels and in many places, cottonwood communities are decadent and have been replaced by mixed community type that consists of cottonwood, peach-leaved willow (Salix amygdaloides), box elder (Acer negundo), American elm (Ulmus americana), green ash (Fraxinus pennsylvanica), Russian olive (Elaeagnus angustifolia) and tamarisk (Tamarix ramosissima) - of which the last four are non-native, introduced species. Other common non-natives include cheatgrass (Bromus tectorum), smooth brome (Bromus inermis), quackgrass (Elytrigia repens), redtop (Agrostis gigantea), Canada thistle (Cirsium arvensis), burning bush (Kochia scoparia), sweetclover (Melilotus spp.), dandelion (Taraxacum officinale), and yellow salsify (Tragopogon dubius). Planted hayfields and irrigated areas can lack trees but still can experience flooding. Reed canary-grass (Phalaris arundinacea) can invade sites with altered hydrology.
- CEGL005007 Baccharis salicina Shrubland
- CEGL001779 Muhlenbergia asperifolia Wet Meadow
- CEGL002708 Panicum obtusum Grassland
- CEGL000659 Populus deltoides - (Salix amygdaloides) / Salix (exigua, interior) Floodplain Woodland
- CEGL002685 Populus deltoides (ssp. wislizeni, ssp. monilifera) / Salix exigua Riparian Woodland
- CEGL005977 Populus deltoides (ssp. wislizeni, ssp. monilifera) / Sporobolus airoides Flooded Woodland
- CEGL002649 Populus deltoides / Carex pellita Floodplain Woodland
- CEGL001454 Populus deltoides / Panicum virgatum - Schizachyrium scoparium Floodplain Woodland
- CEGL005024 Populus deltoides / Pascopyrum smithii - Panicum virgatum Floodplain Woodland
- CEGL000660 Populus deltoides / Symphoricarpos occidentalis Floodplain Woodland
- CEGL005967 Populus deltoides ssp. monilifera / Bouteloua curtipendula Flooded Woodland
- CEGL001203 Salix exigua / Mesic Graminoids Western Wet Shrubland
- CEGL001476 Spartina pectinata Western Wet Meadow
- CEGL001131 Symphoricarpos occidentalis Shrubland
Associated Animal Species
Plains riparian systems provide protected migration routes and abundant nesting, foraging and resources for many wildlife species including white-tailed deer (Odocoileus virginianus), opossum (Didelphis virginiana), muskrat (Ondatra zibethicus), western harvest mouse (Reithrodontomys megalotis) and fox squirrel (Sciurus niger). Cottonwood is an important forage, nesting and cover resource. Beavers (Castor canadensis) use plains cottonwood for food and for buildings dams and lodges. Cottonwood stabilize streambanks and thereby provide fish with thermal cover and protected undercut bank habitat. Plains cottonwood the most important browse species for mule deer in the fall. Abundant shelter, insects and warm temperatures make lowland riparian ecosystems important habitat for reptiles and amphibians and more of these species occur here than do other ecosystems in the region. Reptiles and amphibians most closely associated with riparian corridors include northern cricket frog (Acris crepitans), plains and northern leopard frog (Rana blairi and R. pipiens), smooth green snake (Liochlorophis vernalis), northern water snake (Nerodia sipedon), and common garter snake (Thamnophis sirtalis); the red-spotted toad (Bufo punctatus) and canyon treefrog (Hyla arenicolor) occur primarily in riparian zones in rocky canyon bottoms.
In Northeastern Colorado Plains cottonwood stands provide habitat for 82% of all of the bird species breeding in northeastern Colorado. Birds closely associated with cottonwoods on the Great Plains of Colorado include Wild Turkey (Meleagris gallopavo), Bald Eagles (Haliaeetus leucocephalus), Red-headed Woodpecker (Melanerpes erythrocephalus) which nest in cottonwood, Golden Eagles (Aquila chrysaetos), Lewis's Woodpecker (Melanerpes lewis) which breed predominantly in old decadent cottonwoods, Bullock's Oriole (Icterus bullockii), Yellow Warbler (Setophaga petechia), Western Kingbird (Tyrannus verticalis), Great Horned Owl (Bubo virginianus), Eastern Kingbird (Tyrannus tyrannus), American Kestrel (Falco sparverius), and Blue Grosbeak (Passerina caerulea).
Dynamic Processes
In a natural condition, this system often occurs as a mosaic of multiple communities with variable age-classes of woody species and floodplain features. 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 dominance by upland grasses and weedy forbs tolerant of low soil moisture.
Fluvial processes play an integral role in the dynamics of Western Great Plains Riparian systems. The nature of these processes is often indicated by channel morphology. Meandering occurs in shallow gradient channels with low flow variability and fine-particled substrate, whereas braided channels have steep gradients, high flow variability and sediment dominated by coarse-particled material. Alluvial streams continually change their position as a consequence of hydraulic forces that result in bank erosion and sediment deposition which causes gradual, downstream movement of the overall stream pattern and a complete reworking of the floodplain/riparian zone.
Management
Riparian areas in the Colorado Western Great Plains have been substantially altered by anthropogenic land use and resource management. Alteration of natural hydrology regimes by dams, diversions, ditches, roads, and groundwater usage have considerably changed channel morphology, riparian vegetation, and natural processes from pre-settlement conditions. When systems lack natural flooding due to human management, vegetation responds by shifting from wetland/riparian species to upland species, and systems may become dominated by later-seral communities due to the inability of pioneer species like cottonwoods and willow to establish. The floodplain feature can iteratively contract as channels downcut, and while wetland area may concentrate in the channel, overall site hydrology is reduced. Many systems in the plains are connected to groundwater, and as water is concentrated and stored in dams, and as ranching and agricultural adjacent land uses accumulate, underground water reserves are being replenished less and less causing stream flow loss. Climatic drought also impacts moisture conditions in the unsaturated zones, and decreases streamflow and water table recharge. Increased nutrients from agricultural runoff can alter species composition by prompting aggressive, invasive species to displace native species. Heavy grazing impacts vegetation establishment and composition as well as bank conditions and erosion patterns. In addition, non-native woody species such as saltcedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) are replacing willow and cottonwood stands throughout the plains. Lastly, the removal of beaver (Castor canadensis) has reduced habitat diversity and stability as channels incise, snowmelt flood peaks increase, flood-related sediment transport increases and riparian and slow-velocity habitats are lost.
Colorado Version Authors
- Colorado Natural Heritage Program Staff: CNHP staff: Dee Malone, Joanna Lemly, Karin Decker, Cat Wiechmann, Laurie Gilligan, Sarah Marshall
References
- Carsey, K., G. Kittel, K. Decker, D. Cooper, and D. Culver. 2003. Field guide to the wetland and riparian plant associations of Colorado. Prepared for the Colorado Department of Natural Resources, Denver, CO by the Colorado Natural Heritage Program, Fort Collins, CO.
- Colorado Partners in Flight. 2000. Land Bird Conservation Plan. http://www.blm.gov/wildlife/plan/pl-co-10.pdf.
- Gage, E. and D.J. Cooper. 2007. Historic Range of Variation Assessment for Wetland and Riparian Ecosystems, U.S. Forest Service, Region 2. Department of Forest, Rangeland and Watershed Stewardship, Colorado State University, Fort Collins, CO.
- Kittel, G., E. Van Wie, M. Damm, R. Rondeau, S. Kettler, A. McMullen, and J. Sanderson. 1999. A Classification of Riparian Wetland Plant Associations of Colorado: A Users Guide to the Classification Project. Colorado Natural Heritage Program, College of Natural Resources, Colorado State University, Fort Collins, Colorado.
- Mitsch, W.J. and J.G. Gosselink. 2000. Wetlands, 3rd ed. John Wiley and Sons. New York.
- Mutel, C.F. and J.C. Emerick. 1992. From grassland to glacier: The natural history of Colorado. Johnson Books, Boulder, Colorado.
- Naiman, R.J., J.M. Melillo, and J.E. Hobbie. 1986. Ecosystem Alteration of Boreal Forest Streams by Beaver (Castor canadensis). Ecology 67:1254-1269.
- Pederson, G.T., S. T. Gray, C. A., Woodhouse, J. L. Betancourt, D. B. Fagre, J.S. Littell, E. Watson, B. H. Luckman, L. J. Graumlich. 2011. The Unusual Nature of Recent Snowpack Declines in the North American Cordillera. Science, 15 July 2011:Vol. 333 no. 6040. Pp.332-335.
- Pepin, D.M., N. L. Poff and J.S. Baron. 2002. Ecological Effects of Resouce Development in Running Waters. Pp 113-132 in J.S. Baron, ed. Rocky Mountain Futures: An Ecological Perspective, Island Press.
- Rondeau, R.J. 2001. Ecological system viability specifications for Southern Rocky Mountain ecoregion. First Edition. Colorado Natural Heritage Program, Colorado State University, Fort Collins, CO. 181 pp.
- Windell, J.T. 1992. Stream, Riparian and Wetland Ecology. University of Colorado, Boulder, CO. 222 pp.
- Wohl, E.E. 2001. Virtual rivers: Lessons from the mountain rivers of the Colorado Front Range. New Haven, Conn.: Yale University Press.