Question: The Tongue River Dam impounds a  tributary that is approximately how many miles long?
Context: Great Falls Dam is a hydroelectric dam on the Caney Fork, straddling the county line between White County and Warren County in the U.S. state of Tennessee.  It is the only dam outside the Tennessee River watershed owned and operated by the Tennessee Valley Authority.  The dam impounds the 1830 acre Great Falls Lake, and its tailwaters feed into Center Hill Lake.  The completion of Great Falls Dam in 1917 was an engineering triumph, marking the first successful attempt to impound the volatile and flood-prone Caney Fork.  The dam is also notable for its design, utilizing a mostly underground conduit to carry water from the reservoir via a tributary to the Power House 0.75 mi downstream from the dam. Fontenelle Dam was built between 1961 and 1964 on the Green River in southwestern Wyoming.  The 139 ft high zoned earthfill dam impounds the 345360 acre.ft Fontenelle Reservoir.  The dam and reservoir are the central features of the Seedskadee Project of the U.S. Bureau of Reclamation, which manages the Fontenelle impoundment primarily as a storage reservoir for the Colorado River Storage Project.  The dam suffered a significant failure in 1965, when the dam's right abutment developed a leak.  Emergency releases from the dam flooded downstream properties, but repairs to the dam were successful.  However, in 1983 the dam was rated "poor" under Safety Evaluation of Existing Dams (SEED) criteria, due to continuing seepage, leading to an emergency drawdown.  A concrete diaphragm wall was built through the core of the dam to stop leakage. Melton Hill Dam is a hydroelectric dam on the Clinch River just south of Oak Ridge, Tennessee, United States.  The dam is operated by the Tennessee Valley Authority, which built the dam in the early 1960s to extend the Tennessee Valley's continuous navigation channel up the Clinch as far as Clinton and to increase TVA's overall power-generating capacity.  The dam impounds the 5470 acre Melton Hill Lake, and is the only TVA tributary dam serviced by a navigation lock. Gomal Zam Dam (Urdu: ‎ ) is a multi-purpose gravity dam in South Waziristan Agency of Federally Administered Tribal Areas, Pakistan.  The dam impounds the Gomal River, a tributary of the Indus River, at Khjori Kach, where the Gomal River passes through a narrow ravine.  The purpose of the dam is irrigation, flood control and hydroelectric power generation.  Construction of the dam began in August 2001 and was completed in April 2011.  The powerhouse was completed in March 2013 and electricity production started in August 2013.  The dam was officially inaugurated on 12 September 2013 by Minister for Water and Power Khawaja Muhammad Asif, along with the US Ambassador Richard G. Olson and Khyber Pakhtunkhwa Governor Shaukatullah Khan. The Ubol Ratana Dam (pronounced: "Ubon Rat"), formerly known as the "Phong Neeb Dam", is a multi-purpose dam in tambon Khok Sung, Ubolratana District, approximately 50 km north of Khon Kaen, Khon Kaen Province, Thailand.  It was the first hydroelectric power project developed in Thailand's northeastern area of Isan.  The dam impounds the Nam Phong, which flows into the Chi River and thence to the Mun River, a tributary of the Mekong River.  The dam was given its current name by royal permission in 1966, in honour of princess Ubol Ratana, the eldest child of King Bhumibol Adulyadej. Crystal Dam is a 323 ft double curvature, concrete thin arch dam located six miles downstream from Morrow Point Dam on the Gunnison River in Colorado, United States.  Crystal Dam is the newest of the three dams in Curecanti National Recreation Area; construction on the dam was finished in 1976.  The dam impounds Crystal Reservoir.  Crystal Dam and reservoir are part of the Bureau of Reclamation's Wayne N. Aspinall Unit of the Colorado River Storage Project, which retains the waters of the Gunnison River and its tributaries for agricultural and municipal use in the American Southwest.  The dam's primary purpose is hydroelectric power generation. Lake Keowee is a man–made reservoir in the United States in the state of South Carolina.  It was created to serve the needs of a power utility (Duke Energy) as well as public recreational purposes.  It is approximately 26 mi long, 3 mi wide, with an average depth of 54 ft , and a shoreline measured at 300 mi in total, and is approximately 800 ft above sea level.  It began in 1971 as a massive demolition and building project, including the construction of two large dams––Keowee Dam and Little River Dam––and covered 18372 acre of the state.  The lake collects or "impounds" waters from the Keowee River and the Little River and others, and the outflows below the respective dams join to form the Seneca River which flows into the larger Savannah River.  Lake water helps to cool Duke Energy's three nuclear reactors located at the Oconee Nuclear Generating Station.  In addition, the force of falling water through gravity helps generate hydroelectric power.  The Keowee Hydro Station generates 158 megawatts from the lake's outflows.  In addition, Lake Keowee has been touted as a recreational destination for fishing, boating, swimming, sailing, kayaking and other watersports, and the lake has been described as having pure and clean water.  The name "Keowee" (ᎨᎣᏫ) is a Cherokee name roughly translated as "place of the mullberries."  The former Keowee River, which was inundated by Lake Keowee, had been part of the Cherokee Lower Towns region, and Keowee Town had been located on the bank of the Keowee River. The Hopkinton Dam and Spillway is a historic site at the eastern end of Hopkinton Reservoir in Hopkinton State Park in Ashland, Massachusetts.  The western end of the reservoir lies in the town of Hopkinton.  The reservoir was constructed between 1891 and 1894 as part of the city of Boston's second major water works project, the impoundment of significant portions of the Sudbury River watershed.  The Hopkinton Dam impounds Indian Brook, a Sudbury River tributary, creating the reservoir to its south.  The core of the dam is concrete, with earthen embankments that are bermed on the water side, with rip-rap below.  The spillway is at the northern end of the dam, and is a 650 ft series of steps lined with granite set in concrete.  Unlike other dams in the system, this one apparently never had gatehouses built above the chambers from which water flow is controlled. The Tongue River Dam (National ID # MT00002) is a dam in Big Horn County, Montana, a few miles north of the Wyoming state border.  It impounds the Tongue River, creating the Tongue River Reservoir. The Tongue River is a tributary of the Yellowstone River, approximately 265 mi (426 km) long, in the U.S. states of Wyoming and Montana.  The Tongue rises in Wyoming in the Big Horn Mountains, flows through northern Wyoming and southeastern Montana and empties into the Yellowstone River at Miles City, Montana.  Most of the course of the river is through the beautiful and varied landscapes of eastern Montana, including the Tongue River Canyon, the Tongue River breaks, the pine hills of southern Montana, and the buttes and grasslands that were formerly the home of vast migratory herds of American bison.  The Tongue River watershed encompasses parts of the Cheyenne and Crow Reservations.  The Headwaters lie on the Big Horn National Forest, and the watershed encompasses the Ashland Ranger District of the Custer National Forest.
Answer: 265