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The 125-mile long Smith River, a tributary of the Missouri River, is highly valued as an agricultural resource and for its many recreational uses. During a drought starting in about 1999, streamflow was insufficient to meet all of the irrigation demands, much less maintain streamflow needed for boating and viable fish habitat. In 2006, the U.S. Geological Survey, in cooperation with the Meagher County Conservation District, initiated a multi-year hydrologic investigation of the Smith River watershed. This investigation was designed to increase understanding of the water resources of the upper Smith River watershed and develop a detailed description of groundwater and surface-water interactions. A combination of methods, including miscellaneous and continuous groundwater-level, stream-stage, water-temperature, and streamflow monitoring was used to assess the hydrologic system and the spatial and temporal variability of groundwater and surfacewater interactions. Collectively, data are in agreement and show: (1) the hydraulic connectedness of groundwater and surface water, (2) the presence of both losing and gaining stream reaches, (3) dynamic changes in direction and magnitude of water flow between the stream and groundwater with time, (4) the effects of local flood irrigation on groundwater levels and gradients in the watershed, and (5) evidence and timing of irrigation return flows to area streams. Groundwater flow within the alluvium and older (Tertiary) basin-fill sediments generally followed land-surface topography from the uplands to the axis of alluvial valleys of the Smith River and its tributaries. Groundwater levels were typically highest in the monitoring wells located within and adjacent to streams in late spring or early summer, likely affected by recharge from snowmelt and local precipitation, leakage from losing streams and canals, and recharge from local flood irrigation. The effects of flood irrigation resulted in increased hydraulic gradients (increased groundwater levels relative to stream stage) or even reversed gradient direction at several monitoring sites coincident with the onset of nearby flood irrigation. Groundwater-level declines in mid-summer were due to groundwater withdrawals and reduced recharge from decreased precipitation, increased evapotranspiration, and reduced leakage in some area streams during periods of low flow. Groundwater levels typically rebounded in late summer, a result of decreased evapotranspiration, decreased groundwater use for irrigation, increased flow in losing streams, and the onset of late-season flood irrigation at some sites. The effect of groundwater and surface-water interactions is most apparent along the North and South Forks of the Smith River where the magnitude of streamflow losses and gains can be greater than the magnitude of flow within the stream. Net gains consistently occurred over the lower 15 miles of the South Fork Smith River. A monitoring site near the mouth of the South Fork Smith River gained (flow from the groundwater to the stream) during all seasons, with head gradients towards the stream. Two upstream sites on the South Fork Smith River exhibited variable conditions that ranged from gaining during the spring, losing (flowing from the stream to the groundwater) during most of the summer as groundwater levels declined, and then approached or returned to gaining conditions in late summer. Parts of the South Fork Smith River became dry during periods of losing conditions, thus classifying this tributary as intermittent. The North Fork Smith River is highly managed at times through reservoir releases. The North Fork Smith River was perennial throughout the study period although irrigation diversions removed a large percentage of streamflow at times and losing conditions persisted along a lower reach. The lowermost reach of the North Fork Smith River near its mouth transitioned from a losing reach to a gaining reach throughout the study period. Groundwater and surface-water interactions occur downstream from the confluence of the North and South Fork Smith Rivers, but are less discernible compared to the overall magnitude of the main-stem streamflow. The Smith River was perennial throughout the study. Monitoring sites along the Smith River generally displayed small head gradients between the stream and the groundwater, while one site consistently showed strongly gaining conditions. Synoptic streamflow measurements during periods of limited irrigation diversion in 2007 and 2008 consistently showed gains over the upper 41.4 river miles of the main stem Smith River where net gains ranged from 13.0 to 28.9 cubic feet per second. Continuous streamflow data indicated net groundwater discharge and small-scale tributary inflow contributions of around 25 cubic feet per second along the upper 10-mile reach of the Smith River for most of the 2010 record. A period of intense irrigation withdrawal during the last two weeks in Groundwater and Surface-Water Interaction within the Upper Smith River Watershed, Montana, 2006–2010 By Rodney R. Caldwell and Cheryl A. Eddy-Miller 2 Groundwater and Surface-Water Interaction in the Upper Smith River Watershed May was followed by a period (early June 2010 to mid-July 2010) with the largest net increase (an average of 71.1 cubic feet per second) in streamflow along this reach of the Smith River. This observation is likely due to increased groundwater discharge to the Smith River resulting from irrigation return flow. By late July, the apparent effects of return flows receded, and the net increase in streamflow returned to about 25 cubic feet per second. Two-dimensional heat and solute transport VS2DH models representing selected stream cross sections were used to constrain the hydraulic properties of the Quaternary alluvium and estimate temporal water-flux values through model boundaries. Hydraulic conductivity of the Quaternary alluvium of the modeled sections ranged from 3x10-6 to 4x10-5 feet per second. The models showed reasonable approximations of the streambed and shallow aquifer environment, and the dynamic changes in water flux between the stream and the groundwater through different model boundaries. Introduction The Smith River watershed is an important agricultural and recreational area in Meagher and Cascade Counties in west-central Montana (fig. 1). Nearly 35,000 acres of the Smith River watershed are irrigated, primarily with water directly withdrawn from the Smith River and its tributaries (Cannon and Johnson, 2004). Downstream (northward), the agricultural fields are replaced by a scenic canyon that draws thousands of recreationists each year. During a recent drought, which started in about 1999, streamflow was insufficient to meet all of the irrigation demands, much less maintain streamflow needed for boating and viable fish habitat (Montana Department of Natural Resources and Conservation, 2003). Largely in response to the lack of available surface water for irrigation, some irrigators have already switched, or are proposing to switch, from flood irrigation to sprinkler irrigation (Montana Department of Natural Resources and Conservation, 2003). Additionally, some irrigators have considered using groundwater instead of surface water as a source of irrigation water. The effects of these changes in irrigation practices on the Smith River watershed are unknown. In April 2006, the U.S. Geological Survey (USGS), in cooperation with the Meagher County Conservation District (MCCD), began a study of the hydrogeology of the Smith River Watershed. The project was supported through the combined resources of the Montana Department of Natural Resources and Conservation’s (MDNRC) Reclamation and Development Grants Program and the USGS Cooperative Water Program. The study was designed to improve understanding of the groundwater system with an emphasis on groundwater and surface-water interactions through a systematic program of data collection, research, and analysis. The findings of this study can assist water managers with the development of a comprehensive management program for the use and protection of water resources in the Smith River watershed. This report is the second in a series of reports describing the water resources of the Smith River watershed. The first report (Nilges and Caldwell, 2012) is a USGS Open-File report that summarizes the hydrologic data collected and compiled for the hydrogeologic study of Smith River watershed through water year 2010. This report describes the general hydrology and groundwater and surface-water interactions within the upper watershed based on collected data. Purpose and Scope The purpose of this report is to describe the spatial and temporal interactions of groundwater and surface water in the upper Smith River watershed. The description of groundwater and surface-water interactions includes: (1) generalized groundwater-flow direction, (2) the delineation of gaining (flow from the groundwater to the stream) and losing (flow from the stream to the groundwater) reaches of the upper Smith River and selected tributaries, (3) quantification of gains and losses under different hydrologic conditions, (4) the relation between groundwater levels and stream stage, (5) hydraulic properties of the Quaternary alluvium, and (6) estimated water fluxes between groundwater and surface water at selected stream cross sections. Description of Study Area The Smith River is a tributary to the Missouri River with a watershed encompassing approximately 2,000 square miles (mi2) or nearly 1.3 million acres in Meagher and Cascade Counties of west-central Montana. Study efforts were focused primarily on the approximately 1,200 mi2 in the upper watershed above the Tenderfoot Creek drainage (fig. 1). The Smith River watershed lies within the structurally complex Northern Rocky Mountains Physiographic Division described by Fenneman and Johnson (1946), is characterized by somewhat rugged moun

Groundwater and surface-water interaction within the upper Smith River Watershed, Montana 2006-2010