Alternative A does not designate the Chapman Bench area as an ACEC; the BLM would manage the area in accordance with multiple use principles consistent with other resource objectives.
The Chapman Bench area has been closed to mineral entry because the BOR previously administered the land; Alternative A would open the area to mineral entry. Trace quantities of placer gold have been reported in stream sediment and gravel of Big Sand Coulee in the general area of Chapman Bench. Gold in the Big Sand Coulee area occurs as fine flakes and pin‐point sized fragments (Thomas 1965). Managing the area as available for locatable mineral entry could result in adverse impacts to wildlife habitat if speculative placer gold claims were located in the area. The area has a low potential for gypsum and bentonite, which may minimize the potential for development and associated impacts to the values of concern. Under Alternative A, the area is open to mineral leasing with primarily moderate constraints, which could result in adverse impacts to the special status bird species, vegetation, and wildlife habitat values of concern due to surface disturbance and disruption. However, the development potential for oil and gas (9,206 acres of low potential and 14,121 acres very low potential) in the area may minimize the potential for development and associated impacts.
The area is open to mineral materials disposal, and adverse impacts to the values of concern would likely occur on the 5,852 acres where the potential for sand and gravel is high. Beneficial impacts to mineral development from this management would likely occur on this 5,852 acres by allowing disposal of mineral materials in this area, subject to BLM review.
The Chapman Bench area is managed primarily as open to ROW authorizations (18,668 acres), with a smaller area managed as a ROW avoidance/mitigation area (4,694 acres). Standard guidelines related to surface disturbance would apply. Allowing ROW authorizations would lead to surface disturbance and disruption and related adverse impacts to the values of concern, such as the spread of invasive species or the loss of vegetation. Managing the area as primarily open to ROW authorizations would benefit this resource use.
Alternative A manages motorized vehicle use in the Chapman Bench areas as limited to existing roads and trails, which would benefit motorized travel in the area. Travel management would benefit special status bird species, vegetation, and wildlife habitat by restricting off-road driving and damage to habitat or disruption of wildlife.
Under Alternative B, the BLM would manage the Chapman Bench area as an ACEC for the retention, enhancement, and success of the greater sage-grouse, mountain plover, and long-billed curlew. Prohibiting surface-disturbing activities in the Chapman Bench ACEC would restrict and result in adverse impacts to such activities as geophysical exploration and road construction. This restriction would benefit special status bird species and wildlife in the area by limiting the potential for disruptions, habitat fragmentation, or invasive species infestations that would degrade their habitat.
Under Alternative B, restrictions on mineral development would result in greater adverse impacts to these resource uses than under alternatives A or C. The ACEC is withdrawn from appropriation under the mining laws and administratively unavailable for mineral leasing; however, the potential for gypsum and bentonite is low, and the development potential for oil and gas in the area consists of 9,206 acres of low potential and 14,121 acres of very low potential. Due to the low oil and gas development potential, development and resulting impacts would be minimal. The ACEC is closed to mineral materials disposal, and adverse impacts to mineral development would be greatest on the 5,852 acres with high-potential for sand and gravel. Impacts to mineral development would be greatest under Alternative B, because management is the most restrictive. Restrictions and closures of the area to mineral activity would benefit the values of concern in the ACEC by preventing mining-related surface disturbance, habitat fragmentation, and general degradation of the habitat and disturbance of special status species.
Under Alternative B, most of the Chapman Bench ACEC is a renewable energy and ROW avoidance/mitigation area (17,897 acres) and an ROW exclusion area on the remainder (5,430 acres). Of all the alternatives, this management is the most restrictive to future ROW authorizations and the most restrictive of ROW-related surface disturbance and disruption. This management would result in the greatest beneficial impacts to the wildlife and vegetation values of concern.
Alternative B limits motorized vehicle travel in the ACEC to existing roads and trails, and impacts under Alternative B would be the same as those under Alternative A.
Seasonally stipulating, where feasible, vegetative treatments, invasive, nonnative pest species control, fuels management, and maintenance of existing facilities in the Chapman Bench ACEC would protect wildlife and special status species during sensitive times of the year, while still allowing maintenance and treatments to occur.
Alternative C does not designate the Chapman Bench as an ACEC; the BLM would manage the area in accordance with multiple use principles consistent with other resource objectives.
The management of and impacts associated with mineral resources under Alternative C would be similar to those under Alternative A.
Alternative C manages the Chapman Bench area as an avoidance/mitigation area for ROW authorizations (19,664 acres), and manages a smaller area as open to ROW authorizations (3,662 acres). Management under this alternative would result in greater adverse impacts to ROW authorizations than under Alternative A because a larger portion is an ROW avoidance/mitigation area subject to development constraints or additional mitigation and monitoring that could affect construction costs. Such constraints would benefit special status bird species and wildlife that would be adversely affected by such developments. Standard guidelines related to surface disturbance would apply on portions managed as open to ROW authorizations.
Alternative C limits motorized vehicle use primarily to designated roads and trails (23,268 acres). This alternative is the most restrictive to motorized travel and would result in the greatest adverse impacts to travel and transportation management in the area. This alternative represents the smallest potential for travel-related impacts to the values of concern of any of the alternatives.
Under Alternative D, the BLM manages the Chapman Bench area as a Management Area for the retention and success of the greater sage-grouse, mountain plover, and long-billed curlew. The 3,425 acres of BLM-administered surface ownership managed for these values are all within the Alternative B ACEC (23,326 acres) boundaries. The BLM allows surface-disturbing activities across the entire Chapman Bench area, consistent with other resource objectives and standard guidelines for surface-disturbing activities. The adverse and beneficial impacts of prohibiting such activities, as described for Alternative B, would not occur under this alternative.
The Chapman Bench Management Area is withdrawn from appropriation under the mining laws, open to mineral leasing with an NSO restriction, and closed to mineral materials disposal. The larger area proposed as an ACEC under Alternative B is open to locatable mineral entry, open to mineral leasing with moderate constraints, and open to mineral materials disposal. Impacts from the management of mineral uses in the Chapman Bench Management Area would result in adverse impacts to mineral exploration and development and benefits to the values of concern similar to Alternative B. In the larger area designated as an ACEC under Alternative B, impacts to and from mineral development under this alternative would be less beneficial to the values of concern and more beneficial to mineral development.
ROW management and associated impacts across the Chapman Bench area would be similar to those under Alternative C. Under Alternative D, the Chapman Bench Management Area is a renewable energy and ROW avoidance/mitigation area. The larger area designated as an ACEC under Alternative B generally is managed as an ROW avoidance/mitigation area, with a smaller area managed as open to ROW authorizations (3,691 acres).
Under Alternative D, management of and impacts from motorized vehicle use across the entire Chapman Bench area would be the same as under Alternative A.
Similar to Alternative B, under Alternative D the BLM can stipulate, where feasible, treatment and maintenance activities in the Chapman Bench Management Area to protect wildlife, while still allowing maintenance and treatments to occur. In the larger area proposed as an ACEC under Alternative B, the standard guidelines related to surface disturbance and the management of other resource objectives would apply to these activities; therefore, under Alternative D, impacts in this area would be similar to those under alternatives A and C.
Alternative B would designate the Clarks Fork Basin/Polecat Bench West Paleontological Area as an ACEC (23,895 acres); alternatives A, C, and D would not. Although not proposed under Alternative D, a portion of this area falls within the proposed PETM ACEC. The values of concern in the proposed Clarks Fork Basin/Polecat Bench West Paleontological Area ACEC are paleontological resources in the form of mammalian and paleobotanical fossils and geochemical data used in the study of a major Carbon Isotope Excursion recorded during an ancient period of global warming known as the PETM. Scenic and geologic features also are valuable features in this ACEC. Threats to the area include additional surface disturbance from mineral development (e.g., oil and gas, mineral materials, and possible locatable mineral mining), and ROW development, timber extraction, recreational and OHV use, and invasive and nonnative species infestations. These activities threaten habitat for special status species and create disturbances in crucial winter range during sensitive periods. Heavy public recreational use and existing SRPs also threaten the values of concern in the area. Water quality and quantity issues, as a result of surface and groundwater withdrawals and untreated irrigation outflows, also threaten the area.