Columbia Basin Foothill Streamside Woodland

This is a low-elevation riparian system found on the periphery of the mountains surrounding the Columbia River Basin, along major tributaries and the main stem of the Columbia at relatively low elevations. This is the riparian system associated with all streams at and below lower treeline, including permanent, intermittent and ephemeral streams with woody riparian vegetation. These forests and woodlands require flooding and some gravels for reestablishment. They are found in low-elevation canyons and draws, on floodplains, or in steep-sided canyons, or narrow V-shaped valleys with rocky substrates. Sites are subject to temporary flooding during spring runoff. Underlying gravels may keep the water table just below the ground surface and are favored substrates for cottonwood. Large bottomlands may have large occurrences, but most have been cut over or cleared for agriculture. Rafted ice and logs in freshets may cause considerable damage to tree boles. Beavers crop younger cottonwood and willows and frequently dam side channels occurring in these stands. In steep-sided canyons, streams typically have perennial flow on mid to high gradients. Important and diagnostic trees include Populus balsamifera ssp. trichocarpa, Alnus rhombifolia, Populus tremuloides, Celtis laevigata var. reticulata, Betula occidentalis, or Pinus ponderosa. Important shrubs include Crataegus douglasii, Philadelphus lewisii, Cornus sericea, Salix lucida ssp. lasiandra, Salix eriocephala, Rosa nutkana, Rosa woodsii, Amelanchier alnifolia, Prunus virginiana, and Symphoricarpos albus. Grazing is a major influence in altering structure, composition, and function of the system.

Important and diagnostic trees include Populus balsamifera ssp. trichocarpa, Alnus rhombifolia, Populus tremuloides, Celtis laevigata var. reticulata, Betula occidentalis, or Pinus ponderosa. Important shrubs include Crataegus douglasii, Philadelphus lewisii, Cornus sericea, Salix lucida ssp. lasiandra, Salix eriocephala, Rosa nutkana, Rosa woodsii, Amelanchier alnifolia, Prunus virginiana, and Symphoricarpos albus.

This is a low-elevation riparian system found on the periphery of the mountains surrounding the Columbia River Basin, along major tributaries and the main stem of the Columbia at relatively low elevations. This is the riparian system associated with all streams at and below lower treeline, including permanent, intermittent and ephemeral streams with woody riparian vegetation. These forests and woodlands require flooding and some fresh exposed gravel for reestablishment. They are found in low-elevation canyons and draws, on floodplains, or in steep-sided canyons, or narrow V-shaped valleys with rocky substrates. Sites are subject to temporary flooding during spring runoff. Underlying gravels may keep the water table just below the ground surface and are favored substrates for cottonwood. Large bottomlands may have large occurrences, but most have been cut over or cleared for agriculture. Rafted ice and logs in freshets may cause considerable damage to tree boles. Beavers crop younger cottonwood and willows and frequently dam side channels occurring in these stands. In steep-sided canyons, streams typically have perennial flow on mid to high gradients.

The majority of these forests and woodlands require flooding and freshly deposited gravel/sand for seedling establishment. The natural hydrologic cycle in these reaches includes high spring and early summer flow pulses from snowmelt run off and a natural drawdown into late-summer and fall months. Spring and early summer months also see a rise of the underlying alluvial groundwater table as well as natural lowering of the groundwater in late summer into fall months. High flows and flooding scour (removal) and deposit sediments that stimulate growth of cottonwoods and willows, replenish nutrients, move seeds and aquatic organisms (Merritt and Wohl 2002). These processes stimulate and revive riparian ecosystems. Some reaches are supported by groundwater discharge where flood disturbances are less vital to long-term viability.

Conversion of this type has commonly come from conversion to agricultural development, road development, changes in hydrology either by flooding reaches under reservoirs or complete draining of reaches by 100% upstream diversion by dams and other flood-control activities. Historic and contemporary land-use practices have impacted hydrologic, geomorphic, and biotic structure and function of riparian areas in eastern Washington. Human land uses both within the riparian area as well as in adjacent and upland areas have fragmented many riparian reaches which has reduced connectivity between riparian patches and riparian and upland areas. Adjacent and upstream land uses also have the potential to contribute excess nutrients into riparian areas. Reservoirs, water diversions, ditches, roads, and human land uses in the contributing watershed can have a substantial impact on the hydrology regime. Management effects on woody riparian vegetation can be obvious, e.g., removal of vegetation by dam construction, roads, logging, or they can be subtle, e.g., removing beavers from a watershed, removing large woody debris, or construction of a weir dam for fish habitat. Continuous heavy grazing is a major influence in altering structure, composition, and function of the community (Kauffman et al. 2004). In general, excessive grazing by livestock or native ungulates leads to less woody cover and an increase in sod-forming grasses particularly on fine-textured soils. Less palatable species, such as Juncus balticus and Equisetum spp., increase with livestock use. In many areas, Phalaris arundinacea has almost completed displaced native herbaceous vegetation thereby causing a conversion from native to ruderal vegetation. Although the presence of Phalaris is often due to changes in hydrology or physical disturbances, once established it can become a stressor in and of itself. Non-native plants or animals, which can have wide-ranging impacts, also tend to increase with these stressors. All of these stressors have resulted in many riparian areas being incised, supporting altered riparian plant communities, as well as numerous non-native species (WNHP 2011).

In the Pacific Northwest regionally downscaled climate models project increases in annual temperature of, on average, 3.2°F by the 2040s. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), and some models project wetter autumns and winters and drier summers. Increases in extreme high precipitation (falling as rain) in the western Cascades and reductions in snowpack are key projections from high-resolution regional climate models (Littell et al. 2009). Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing. This change will result in less winter snow accumulation, higher winter streamflows, earlier spring snowmelt, earlier peak spring streamflow and lower summer streamflows in rivers that depend on snowmelt (as do most rivers in the Pacific Northwest) (Littell et al. 2009).

Potential climate change effects could include: further reduction in summer flows (Littell et al. 2009); increases in extreme high precipitation events over the next half-century (Littell et al. 2009); earlier high-flow pluses that may negatively affect cottonwood species dominance as their seed production is timed for June-July high flow for distribution onto wet sand and gravel bars (Boes and Strauss 1994, Merritt and Wohl 2002), so cottonwood-dominated streams may shift to other deciduous tree species; drop in groundwater table; and increased fire frequency due to warmer temperatures resulting in drier fuels; the area burned by fire regionally is projected to double by the 2040s and triple by the 2080s (Littell et al. 2009).

Found on the periphery of the northern Rockies in the Columbia River Basin, along major tributaries and the main stem of the Columbia at relatively low elevations.