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J I I I I I I I I I I- I I I I I I I I I O~1792 2. Background The Upper Arkansas River Basin has been intermittently mined since the mid-1800's. Storage of mine waste on hillslopes, within valley bottoms, and along channels resulted in erosion and transport of metals-laden tailings downstream. Currently, the channel perimeter, channel margin, and floodplain environments of the Arkansas River downstream of California Gulch contain mine derived sediment of variable metals content (URS Operating Services, 1997). Transbasin water diversion facilities have imported flows to the Upper Arkansas River Basin since the early-1900's. The existing channel system has been utilized to convey diverted flows such that the hydrology of the Upper Arkansas River and its tributaries has been altered over the last century, Controlled releases out of Turquoise Reservoir into Lake Fork Creek, coupled with flow conveyance through the Mt. Elbert Conduit have further modified project reach hydrology. Additional activities that have affected the geomorphic evolution of the project reach include livestock grazing, flow diversions, and floodplain irrigation. 2.1. The Upper Arkansas River Restoration Project As stated in the Request for Proposals (RFP), the primary goal of the Upper Arkansas River Restoration Project (UARRP) is the restoration of the II-mile project reach to a healthy ecosystem. To achieve this goal, several restoration objectives have been identified by the UARRP Core Group, including: reduction of riverbank erosion, property loss and channel aggradation/degradation; reduction of the impacts of historic mine waste deposits to the aquatic and riparian system; improvement of the riparian zone by re- establishing healthy, naturally occurring riparian plant communities; improvement of fisheries; and minimization of long-term stream channel and riverbank maintenance. . Although the stated objectives of the UARRP Core Group do not define the specific objectives of this investigation, the UARRP restoration objectives have been considered in the development of process-based rehabilitation strategies. 2.2. Project Goals and Objectives This investigation has been conducted as one component of the Upper Arkansas River Restoration Project. The primary goal ofthis investigation is a geomorphic assessment of the II-miles of channel and its floodplain from California Gulch to the mouth of Box Creek. The two main components of the geomorphic assessment include the characterization of the geomorphic history of the project reach, and the assessment of current channel stability, The geomorphic evolution of the reach provides insight as to the physical influences that have driven channel adjustments, and the channel stability assessment describes the current state of those adjustments. The geomorphic assessment has been utilized to estimate the relative potential for tailings entrainment on a subreach basis. From this direct integration of geomorphic process and general tailings distributions, conceptual means of reducing that potential have been developed. This assessment does not address the relative threat posed by individual tailings deposits, which can be variable due to the range of metals concentrations present, but is based on general entrainment potential of identified deposits. Furthermore, while May 7, 1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 5