Question: Mapes Hotel is next to what river that flows northeasterly and is 121 mi long
Context: The Royal River is a small river, 39 mi long, in southern Maine, United States.  The river originates in Sabbathday Lake in New Gloucester and flows northeasterly into Auburn and then southerly through New Gloucester (via the Royal River Reservoir), Gray and North Yarmouth into Casco Bay at Yarmouth.  It is named after William Royall, one of the first European settlers in the area, though the official form of its name omits the second "L". Grayson Creek is a stream in Contra Costa County in northern California that flows northeasterly 7.4 mi from its origin in Briones Regional Park to Pacheco Slough four miles east of Martinez.  Pacheco Slough, in turn, connects to Suisun Bay.  The Grayson Creek subwatershed is part of the Walnut Creek watershed and includes the cities of Pleasant Hill and Pacheco, California. Mapes Hotel was a hotel and casino located in Downtown Reno, Nevada, next to the Truckee River on Virginia Street.  It was built in 1947 and opened on December 17 of that year.  It was the first skyscraper built in the Western United States since the start of World War II.  Built in a distinctive Art Deco style, the hotel was a unique high-rise built to combine a hotel and casino, providing the prototype for modern hotel/casinos. The Severn River is a river in northern Ontario.  The northern Ontario river has its headwaters near the western border of the province.  From the head of the Black Birch River, the Severn River is 982 km long and its drainage basin is 102,800 km² (39,700 mile²), a small portion of which is in Manitoba.  Its source is Deer Lake and flows northeasterly into Severn Lake, then by a second section to Hudson Bay where it ends at Fort Severn. The Truckee River is a river in the U.S. states of California and Nevada.  The river flows northeasterly and is 121 mi long.  The Truckee is the sole outlet of Lake Tahoe and drains part of the high Sierra Nevada, emptying into Pyramid Lake in the Great Basin.  Its waters are an important source of irrigation along its valley and adjacent valleys. The Upper Truckee River is a stream that flows northward from the western slope of Red Lake Peak in Alpine County, California to Lake Tahoe via the Truckee Marsh in South Lake Tahoe, California.  The river flows northeasterly and is 23 mi long.  It is Lake Tahoe's largest tributary. The Gasper River is a 38.6 mi river in southwestern Kentucky, United States.  It flows northeasterly into the Barren River.  It is a rural river; the only town near it is Hadley in Warren County.  It begins in northeast Logan County, and the river also flows through Warren and Logan counties.  Tributaries include Belcher, Salt Lick, Brush, Clear Fork, Westbrook, and Rock House creeks. The Southwest Branch Saint John River (French: "Rivière Saint-Jean Sud-Ouest" ) is a 62.0 mi river in Maine and Quebec.  The branch originates in "Little Saint John Lake" ( ) on the international boundary between Saint-Zacharie, Quebec and Seboomook Lake Township 5, Range 20, WELS.  The branch forms the Canada–United States border as it flows northeasterly to a confluence with the Little Southwest Branch Saint John River in Seboomook Lake Township 9, Range 18, WELS.  The Southwest Branch flows briefly into Quebec ( ) and then through Maine to its confluence with the Baker Branch Saint John River in Seboomook Lake Township 9, Range 17, WELS.  The Southwest Branch finally joins with the Northwest Branch to form the Saint John River. The East Savanna River is a small yet historic stream in Aitkin and Saint Louis counties in the U.S. state of Minnesota.  With a total length of 15.3 mi , the stream rises in Wolf Lake, a small body of water within a spruce bog in Savanna Portage State Park, and flows northeasterly to the Saint Louis River at Floodwood, Minnesota, whence its waters flow through the Great Lakes and the Saint Lawrence River to the Atlantic Ocean.  A few thousand years ago the East Savanna was part of the Mississippi River itself, originating in northeast Minnesota and flowing southwesterly to Big Sandy Lake, from which the great river drained down its present valley to the Gulf of Mexico. Hale Creek is a short stream originating in the foothills of Los Altos Hills, California in Santa Clara County, California, United States.  Its source is in the Rancho San Antonio Open Space Preserve upstream and east of Neary Quarry.  The creek flows northeasterly 4.6 mi through the cities of Los Altos Hills, Los Altos, and Mountain View before joining Permanente Creek.
Answer: Truckee River