Sage Creek in south-central Wyoming is listed as impaired by the U.S. Environmental Protection Agency (USEPA) due to its sediment contribution to the North Platte River. Despite the magnitude of sediment impacts on streams, little research has been conducted to characterize patterns of sediment transport or to model suspended sediment concentration in many arid western U.S. streams. This study examined the relationship between stream discharge and suspended sediment concentration near the Sage Creek and North Platte River confluence from 1998 through 2003. The objectives were to determine patterns of stream discharge and suspended sediment concentration, produce a sediment prediction model, and compare sediment concentrations for the six-year period. Stream discharge and suspended sediment transport responded rapidly to convective storms and spring runoff events. During the study period, events exceeding 0.23 m^sup 3^/s accounted for 92 percent of the sediment load, which is believed to originate from erodible headwater uplands. Further analysis of these data indicates that time series modeling is superior to simple linear regression in predicting sediment concentration. Significant increases in suspended sediment concentration occurred in all years except 2003. This analysis suggests that a six-year monitoring record was insufficient to factor out impacts from climate, geology, and historical sediment storage.

Sediment supply, transport, and deposition may be altered by riparian zone vegetation, which has been shown to reduce suspended sediment in overland flow by more than 90 percent (Pearce et al., 1998a). Coarse grain sediment (63 to 2,000 microns) may be deposited in a foothill riparian zone within the first 10 m from the point of detachment, while smaller material can be suspended longer (Pearce et al., 1998b). Deposited sediment also may be stabilized by vegetation (Clary et al., 1996; Pearce et al., 1997). Because riparian vegetation may cause sediment to be deposited, maintaining a vegetation buffer strip for filtering sediment is important. A riparian zone's continued sediment filtering and stabilization modifies channel bank and floodplain characteristics into sequential and recognizable patterns, or channel succession states (Skinner, 1998). United States Federal land management agencies use channel and floodplain succession states to evaluate riparian zone condition. According to the U.S. Bureau of Land Management (USBLM, 1998), a riparian wetland area is considered to be in proper functioning condition (PFC) when adequate vegetation, landform, or large woody debris is present to: dissipate stream energy associated with 5-year, 10-year, and 20-year events, thereby reducing erosion and improving water quality; filter sediment, capture bedload, and aid floodplain development; improve floodwater retention and ground water recharge; develop root masses that stabilize streambanks against cutting action; develop diverse ponding and channel characteristics to provide the habitat and the water depth, duration, and temperature necessary for fish production, waterfowl breeding, and other uses; and support greater biodiversity. Proper functioning condition is similar to Heede's (1980) description of a stream channel in dynamic equilibrium with a basin's sediment load and flow regime.

Proper functioning condition assessments are qualitative evaluations that determine riparian wetland area condition. During the PFC assessment, trained personnel complete vegetation, soils, and hydrologie condition checklists to evaluate a riparian area's ability to withstand large flow events. The assessments are meant to be used as a communication tool and an appropriate starting point for determining and prioritizing the type and location of needed quantitative inventory or monitoring (USBLM, 1998). Because PFC assessments do not determine water quality, it is possible for a stream to be rated as properly functioning and also be a significant contributor of nonpoint source pollutants; basically PFC assessments only rate a riparian area's resiliency to withstand high flow events based on favorable physical and biological characteristics.

A stream that is in PFC therefore should be capable of transporting the total of its suspended sediment load downstream under its long term flow regime without a noticeable alteration in channel conditions. A stream channel configuration that is in dynamic equilibrium with its flow regime and sediment load, or PFC, should have limited sediment storage, resulting in an increased ability to transport nonpoint source pollution (Skinner, 2003). Riparian zone importance and function along stream corridors is well documented (Kauffman and Krueger, 1984; Gregory et al., 1991; Dillaha and Inamdar, 1997; Naiman and Decamps, 1997), and sediment is a significant nonpoint source pollutant associated with streamflow (USEPA, 1989, 1995). Initiating best management practices (BMPs) for reducing sediment in streams has been encouraged by the USEPA and state regulatory agencies.