Characterize the Watershed Core Topics and Questions Soil Resources
Total Page：16
File Type：pdf, Size：1020Kb
Step 1: Characterize the Watershed Purpose To identify the dominant physical, biological, and human processes and features of the watershed that affect ecosystem function or condition. To relate these features and processes with those occurring in the river basin or province. To provide the watershed context for identifying elements that need to be addressed in the analysis. To identify, map, and describe the most important land allocations, plan objectives, and regulatory constraints that influence resource management in the watershed. Core Topics and Questions Soil Resources ¾ What is the hierarchy of ecological units in the watershed, and what is the spatial relationship of these units? What are the inherent stability and erosion processes? The Blue Creek watershed is located within the Dry Domain of the Temperate Steppe Regime Mountains Division of the Southern Rocky Mountain Steppe-Open Woodland-Coniferous Forest Province of the Overthrust Mountains Section of the Centennial Mountains and Island Park Subsection, outlined in the National Hierarchical Framework for Ecological Units (USDA Forest Service 1994). Descriptions of these different levels of ecological units are found in “Targhee National Forest Subsections and Landtype Associations” (USDA-FS 1998). There are three landtype associations nested within the Centennial Mountains and Island Park Subsections. These are identified as Beaverhead and Centennial Mountains- Conifer Forest, Island Park Plains and Falls River Tablelands-Conifer Forest, and Figure 1 Blue Creek Watershed vicinity map. Island Park Tablelands-Conifer Forest. Figure 1 shows the watershed boundary overlain by the Subsections. Blue Creek Watershed Analysis 4 Physiography The landforms within the watershed consist mainly of caldera floor and scarp, dissected tablelands, foothills, and mountain slopes. Elevation ranges from 9,800 feet at the summit of Sawtell Mountain to 6,200 feet at the mouth of Henrys Fork. Slopes range from nearly level in riparian areas to over seventy percent on mountain sideslopes. Drainage patterns are dendritic with slight to moderate dissections. Some drainages are structurally controlled by bedrock. Aspects are generally southeast and southwest because of the trending slopes of the Island Park Caldera toward the Henrys Fork and the large mountains in the northwest corner of the watershed that form the Centennial Range (Hamilton 1965). Geology Geological information for the watershed was collected from several publications and maps (Mitchel et al. 1979; Hamilton 1965; and Ross et al. 1967). The Island Park Caldera is an elliptical collapsed structure in the center of a rhyolite shield. Surficial geology in the watershed consists mainly of volcanic rocks of welded rhyolite tuff, basalt flows, and flows from the Madison and Pitchstone Plateaus of the Yellowstone Plateau. The rim of the caldera is the remnant of a broad shield of rhyolite in which ash flows predominate over ash falls and lava flows. Primary geologic formations, identified by the Idaho Department of Lands, Bureau of Mines and Geology, which formed the caldera and rim are post-caldera rhyolite ash flows and olivine basalt. The age of these geologic materials are Quaternary and Tertiary. Many ridges and mountain sideslopes in this watershed have exposed volcanic bedrock. Approximately 11,000 acres (20%) in the watershed have the potential for mass instability; however, no landslides have been mapped. Ecological Units and Soils This watershed has few areas that have unstable landforms that show active landslides (USDA-FS 1997a). For the purposes of this characterization, three major landform groupings 1.) mountains and foothills, 2.) caldera floors and dissected tablelands/basalt plains, and 3.) terraces, outwash plains and floodplains are discussed. This is followed by a more detailed description of the 6 main ecological landtypes (of the 23 mapped in the area). Soils that formed from the geologic parent materials listed above have base saturation that provides relatively high natural fertility. Because many of the soils in this watershed formed from geology containing rhyolite and basalt, they have a dominance of sand and gravel in the profile. Clayey soils are usually associated with alluvial landforms. These kinds of soils have high erosion potential and may produce high levels of sediment when erosive conditions occur. Following in Figure 2 is an Ecological Unit map of the watershed including a table containing acres of each ecological unit on National Forest System lands. No information is shown for private land. Blue Creek Watershed Analysis 5 Figure 2 Ecological Unit map of the Blue Creek Watershed. Ecological Units on Mountains and Foothills On the National Forestland in the watershed, mountain and foothill landforms make up approximately 27,500 acres (47%). Soils that formed on the mountains and ridges are moderately deep to very deep (20”->60”) with some rock outcrop located on the steeper mountain sides. They are mainly Rhylow, Fitzwill, Edgway, Cryumbrepts, Yodal and Koffgo soils. Although these soils formed on steeper landforms, they have moderate to low erosion potential and have soil loss tolerance levels ranging from 2 to 5 tons per acre per year. They have higher potential to erode when they lose their protective ground cover. The potential is highest for mass movement on these landforms when soils become saturated with water. Ecological Units on Caldera floors and Dissected Tablelands/Basalt Plains Soils that formed on the caldera floor and dissected tablelands/basalt plains are generally moderately deep to very deep (20 to >60”) and are well drained. They are dominantly vegetated with conifers, big sagebrush, aspen and mountain shrubs. These soils have less potential to erode than those formed on the mountains and ridges because they formed on slopes less than 40 percent. Maintenance of ground cover on these soils is important to maintain stable conditions. They have high range productivity potential and support a variety of uses including grazing, recreation and wildlife habitat. Compaction of these Blue Creek Watershed Analysis 6 soils occurs when areas are used heavily from dispersed recreation, grazing and off-road vehicle use. Erosion from roads in this area is notable because ruts are often formed from vehicles when they are wet and channel sediment into the streams. Road maintenance is often required to reduce these impacts. Ecological Units on Terraces, Outwash Plains and Floodplains Soils that formed on terraces, outwash plains and floodplains are the most productive in the watershed. They are almost always very deep (>60”) and are influenced by wetness during some period of the year. Riparian vegetation such as willows and sedges grow in most of the riparian soils. Soils on terraces may be affected by grazing livestock and wildlife, recreation, roads and influences from upland conditions. Some areas are susceptible to down-cutting which often lowers the water table. When this occurs, riparian vegetation is gradually replaced by upland species such as sagebrush. Most riparian soils appear to be in a productive condition in the watershed but some gully erosion and trampling in riparian areas has been observed. Hydrology ¾ What are the dominant hydrologic characteristics and other notable hydrologic features and processes in the watershed? ¾ What are the basic morphological characteristics of stream valleys and segments and the general sediment transport and deposition processes in the watershed? The Blue Creek Watershed is a 58,492 acre watershed, located within the Henrys Fork basin, between Henrys Lake and Island Park Reservoir. Henrys Fork basin extends from the Idaho border to the Henrys Fork confluence with the South Fork Snake River, north of Idaho Falls, encompassing about 3220 square miles. Average river outflow is about 2,100 cubic feet per second, or about 1,407,000 acre feet per year (Idaho Water Resource Board 1992). Landscape Characteristics: Ecological Landtype Units The physical features of each sub-watershed have been described by the Targhee National Forest Ecological Unit Inventory (USFS 1999) (see Soils section). Each Ecological Unit (EU) is described by drainage patterns, physiography, soils and climate. Within the assessment area, there are six Ecological Units. These Ecological Units are: 1316 – This EU is delineated from Sawtell peak north and west. It is the extremely steep topography on and directly adjacent to the peak. The EU consists of glacial troughs, cirques and on the north side of topographically dominating peaks and ridges. Very steep slopes support open canopy mixed conifer forests that are frequently dissected by avalanche chutes, rock outcrops and rubble land. The soils erode and compact easily when disturbed, which could result in moderate to high rates of erosion and sediment delivered to stream channels from disturbed areas. The soils are within hydrologic groups B and C, which means they have a moderate to slow infiltration rate when thoroughly wet and have a moderate to slow rate of water transmission through the soil Blue Creek Watershed Analysis 7 profile. This would result in moderate amounts of runoff from snowmelt and precipitation events. Annual precipitation averages 32 inches. 1270 – This EU is delineated from Sawtell Peak south and east. Within this unit are the Yale headwaters, East Fork Hotel headwaters, Arrange Creek headwaters, West Fork Hotel Creek headwaters, and east slope Blue Creek headwaters. The EU is a transitional area