Question: Laton Alton Huffman was an American photographer who worked at an Army post  situated on the south bank of the Yellowstone River, at the mouth of what?
Context: Silver Cord Cascade is a horsetail type waterfall on Surface Creek, a tributary of the Yellowstone River in Yellowstone National Park.  Surface Creek flows out of Ribbon Lake off the South rim of the Grand Canyon of the Yellowstone and plunges 1200 ft to the Yellowstone River.  It is considered the tallest waterfall in Yellowstone. Fort Brooke was a historical military post situated on the east bank (at the mouth) of the Hillsborough River in present-day Tampa, Florida.  The fort was established in 1824, soon after Florida was acquired by the United States from Spain.  It was an important post during the Seminole Wars and of lesser importance during the United States Civil War.  It was decommissioned in 1883, and the land was eventually sold to private interests. The Gardner River (also known as the Gardiner River) is a tributary of the Yellowstone River, approximately 25 mi long, in northwestern Wyoming and south central Montana in the United States.  The entire river is located within Yellowstone National Park.  It rises on the slope of Joseph Peak, Gallatin Range in the northwestern part of the park, and winds southeast through Gardner's Hole, a broad subalpine basin which is a popular trout fishing location.  Angling on the Gardner is governed by Yellowstone National Park fishing regulations.  After merging with Panther Creek, Indian Creek and Obsidian Creek, it then turns north and flows through a steep canyon where it cuts through a basaltic flow from approximately 500,000 years ago known as Sheepeater Cliffs. Below Sheepeater, Glen Creek out of Golden Gate Canyon and Lava Creek out of Lava Creek Canyon join the Gardner near Mammoth Hot Springs.  The river crosses the 45th parallel in Gardner Canyon and is also home to a popular hot spring known as "The Boiling River".  The river continues north through Gardner Canyon and empties into the Yellowstone near Gardiner, Montana. Fort Keogh is a former United States Army post located on the western edge of today's Miles City, Montana.  It is situated on the south bank of the Yellowstone River, at the mouth of the Tongue River. The Chittenden Memorial Bridge is a 120 ft concrete and steel arch bridge across the Yellowstone River just upstream from the Upper Yellowstone Falls in Yellowstone National Park.  First constructed in 1903 as a Melan arch bridge by park engineer Captain Hiram M. Chittenden of the US Army Corps of Engineers, the bridge was known as Chittenden Bridge from 1912 to 1963 when it was replaced with a more modern structure.  This bridge provides road access from the Grand Loop Road to the secondary road on the south rim of the Grand Canyon of the Yellowstone that allows visitors to see the upper and lower Yellowstone Falls from the south rim. Laton Alton Huffman (October 31, 1854 – February 28, 1931) was an American photographer of Frontier and Native American life.  Born in Winneshiek County, Iowa, he spent most of his life photographing the area around his Montana home.  Having initially worked at Fort Keogh, he started to sell prints of his glass plate negatives. The Yellowstone expedition was a frontier expedition authorized in 1818 by United States Secretary of War John C. Calhoun to establish a military fort or outpost near present-day Bismarck, North Dakota at the mouth of the Yellowstone River.  Sometimes called the Atkinson-Long Expedition, it led to the creation of Fort Atkinson (Nebraska), the first United States Army post to be established west of the Missouri River, but was otherwise a costly failure, stalling near Council Bluffs, Iowa. Uncle Tom's Trail is a steep stairway descent from the south rim of the Grand Canyon of the Yellowstone to a viewpoint near the base of the Lower Yellowstone Falls in Yellowstone National Park.  The trail was constructed in 1898 by park concessionaire, "Uncle Tom" H. F. Richardson when the Department of the Interior granted Richardson a permit to operate a ferry across the Yellowstone River.  He ferried park visitors across the Yellowstone River above the current site of the Chittenden Memorial Bridge then escorted them to the trail and they traveled down to the base of the Lower Falls via ladders and ropes.  Upon their return, visitors were provided a picnic lunch on the south rim of the canyon before returning via the ferry.  In 1903 when the original Chittenden Bridge was built, Richardson's ferry business began to decline.  In 1905, when the government built a wooden stairway, visitors were increasingly unwilling to pay Richardson the $1 fee for the tour to the base of the falls.  1906 was the last year he operated tours in the canyon. The Nez Perce in Yellowstone Park was the flight of the Nez Perce Indians through Yellowstone National Park between August 20 and Sept 7, during the Nez Perce War in 1877.  As the U.S. army pursued the Nez Perce through the park, a number of hostile and sometimes deadly encounters between park visitors and the Indians occurred.  Eventually, the army's pursuit forced the Nez Perce off the Yellowstone plateau and into forces arrayed to capture or destroy them when they emerged from the mountains of Yellowstone onto the valley of the Yellowstone River. Fort Alexander -"Sagkeeng" Ojibway (mouth of the river) is a community in Manitoba, Canada, located on the Sagkeeng First Nation, on the south bank of the Winnipeg River.  The Sagkeeng area, or the mouth of the Winnipeg River, was originally settled with native camps used for fishing, hunting, and trade.  During the fur trade era, La Vérendrye built a trading post, named Fort Maurepas, on the north side of the river; this post was abandoned near the end of the French period.  In the year 1792, a clerk for the North West Company, Toussaint Lesieur, built a post on the south side, which became an important provisioning post for the canoe brigades.  Bags of pemmican, brought from the North West Company's posts on the upper Assiniboine, were stored here and taken as needed by the canoe brigades passing between Grand Portage (later, Fort William) and the far northwest.  This post was usually referred to as Fort Bas de la Rivière, because of its location at the bottom of Winnipeg River, and it seems to have functioned as the capital of the Company's Lake Winnipeg district.  The Hudson's Bay Company operated its own post here for a few years between 1795 and 1801.  In 1807, the North West Company partner Alexander MacKay rebuilt the post on a nearby site.  Beginning in 1808, the new post was known as Fort Alexander.  After the Northwest and Hudson's Bay Companies merged in 1821, Fort Alexander was operated as a trading post for the natives in the region.
Answer: the Tongue River