Question: The refuge for a historically significant herd of free-roaming Mustangs is close to what lake? 

Context paragraphs:
Pryor Mountains Wild Horse Range: The Pryor Mountains Wild Horse Range is a refuge for a historically significant herd of free-roaming Mustangs, feral horses colloquially called "wild horses", located in the Pryor Mountains of Montana and Wyoming in the United States.  The range has an area of 39650 acre and was established in 1968 along the Montana–Wyoming border as the first protected refuge dedicated exclusively for Mustangs.  It was the second feral horse refuge in the United States.  About a quarter of the refuge lies within the Bighorn Canyon National Recreation Area.  A group of federal agencies, led by the Bureau of Land Management, administers the range.

Medicine Lake National Wildlife Refuge: Medicine Lake National Wildlife Refuge is a 31533 acre National Wildlife Refuge located in the northeastern region of the U.S. state of Montana.  The refuge is part of the Medicine Lake National Wildlife Refuge Complex which also includes the Northeast Montana Wetland Management District (WMD) and Lamesteer National Wildlife Refuge.  Medicine Lake is from the Assiniboine description of the lake, "Bda wauka" (medicine water).  The refuge was established in 1935 to protect habitat for various wildlife and especially migratory birds.  The refuge is managed by the U.S. Fish and Wildlife Service, an agency of the U.S. Department of the Interior.

Old Sandwich Town: Old Sandwich Town (Olde Sandwich Towne) is located along the Canada–US border of the Detroit River, and was established in 1797.  It is considered one of the oldest, most historically significant settlements in Ontario and has been the site of several historically significant events in Ontario's history.  Many historic buildings remain in Old Sandwich Town and the area hosts an annual festival to celebrate its heritage.  The area is now a neighbourhood of the city of Windsor.  Explore the self-guided tour to learn more about the history and heritage of the former town of Sandwich.

Pryor Mountain Mustang: The Pryor Mountain Mustang is a substrain of Mustang considered to be genetically unique and one of the few strains of horses verified by DNA analysis to be descended from the original Colonial Spanish Horses brought to the Americas by the Spanish.  They live on the Pryor Mountains Wild Horse Range located in the Pryor Mountains of Montana and Wyoming in the United States, and are the only Mustang herd remaining in Montana.  They are protected by the Wild and Free-Roaming Horses and Burros Act of 1971 (WFRHBA) and managed by the Bureau of Land Management (BLM), who has set the optimum herd number at 120 animals.  Genetic studies have revealed that the herd exhibits a high degree of genetic diversity and a low degree of inbreeding, and BLM has acknowledged the genetic uniqueness of the herd.  Pryor Mountain Mustangs are relatively small horses, exhibit a natural ambling gait, and domesticated Pryor Mountain Mustangs are known for their strength, sure-footedness and stamina.

Bighorn Canyon National Recreation Area: Bighorn Canyon National Recreation Area is a national park unit established by an act of Congress on October 15, 1966, following the construction of the Yellowtail Dam by the Bureau of Reclamation.  It straddles the border between Wyoming and Montana.  The dam, named after the famous Crow leader Robert Yellowtail, harnesses the waters of the Bighorn River by turning that variable watercourse into Bighorn Lake.  The lake extends 71 mi through Wyoming and Montana, 55 mi of which lie within the national recreation area.  About one third of the park unit is located on the Crow Indian Reservation.  Nearly one-quarter of the Pryor Mountains Wild Horse Range lies within the Bighorn Canyon National Recreation Area.

Lamar Buffalo Ranch: The Lamar Buffalo Ranch was created to preserve one of the last free-roaming bison (buffalo) herds in the United States.  The ranch was established in 1907 when 28 bison were moved from Fort Yellowstone to the Lamar Valley in the northeast portion of the park.  The herd was maintained as a semi-domesticated source of additional bison to enhance the park's natural herd.  The ranch supported bison ranching till the 1950s.  As the ranched herd increased in size, it was released to the open range and it interbred with the wild herd.  The ranch continued to be used to produce hay to feed the bison in the winter until the 1950s.  Because the winter feeding program maintained the herd at artificially high levels and distorted the herd's distribution in the park, it fell from favor as the National Park Service adopted wildlife management policies that interfered as little as possible with each species' natural ecology.

Lake Merritt: Lake Merritt is a large tidal lagoon in the center of Oakland, California, just east of Downtown.  It is surrounded by parkland and city neighborhoods.  It is historically significant as the United States' first official wildlife refuge, designated in 1870, and has been listed on the National Register of Historic Places since 1966.  The lake features grassy shores; several artificial islands intended as bird refuges; an interpretive center called the Rotary Nature Center; a boating center where sailboats, canoes and rowboats can be rented and classes are held; and a fairy tale themed amusement park called Children's Fairyland.  A popular walking and jogging path runs along its perimeter.  The circumference of the lake is 3.4 mi and its area is 155 acre .

Two Brothers Rocks–Dudley Road Historic District: The Two Brothers Rocks–Dudley Road Historic District encompasses a historically significant rural area of Bedford and Billerica, Massachusetts.  The district covers 230 acre of predominantly rural and residential property, along Dudley Street, a narrow, winding road that was laid out in colonial days.  It also includes a significant amount of conservation land, including local, state, and federal lands.  The federal lands of the Great Meadows National Wildlife Refuge which line the banks of the Concord River, include the "Two Brothers Rocks", which were used to mark a land boundary between grants given to early Massachusetts settlers John Winthrop and Thomas Dudley, and which featured as boundary markers into the 20th century.

Audubon National Wildlife Refuge: Audubon National Wildlife Refuge is a 14739 acre National Wildlife Refuge in the U.S. state of North Dakota.  The refuge is managed by the U.S. Fish and Wildlife Service and is the centerpiece of the Audubon National Wildlife Refuge Complex, which includes numerous other refuges in the region.  Originally designated as the Snake Creek National Wildlife Refuge in 1955, the refuge was renamed in 1967 in honor of the artist and naturalist John James Audubon.  Most of the refuge area is a lake known as Audubon Lake which is managed by the U.S. Army Corps of Engineers.

Mustang: The mustang is a free-roaming horse of the American west that first descended from horses brought to the Americas by the Spanish.  Mustangs are often referred to as wild horses, but because they are descended from once-domesticated horses, they are properly defined as feral horses.  The original mustangs were Colonial Spanish horses, but many other breeds and types of horses contributed to the modern mustang, resulting in varying phenotypes.  In the 21st century, mustang herds vary in the degree to which they can be traced to original Iberian horses.  Some contain a greater genetic mixture of ranch stock and more recent breed releases, while others are relatively unchanged from the original Iberian stock, most strongly represented in the most isolated populations.

Supporting paragraphs: Bighorn Canyon National Recreation Area Pryor Mountains Wild Horse Range
Answer: Bighorn Lake