Devil's Lake (often written as Devils Lake) is a freshwater lake in the U.S. state of North Dakota. It is the largest natural body of water in the state and the second-largest body of water in North Dakota after the artificially created Lake Sakakawea. In 2006, it reached a historical high elevation of , an area of 216 mi² (560 km²), and a volume of 2.7 million acre-feet (3.3 km³).
The city of Devils Lake, North Dakota takes its name from the lake.
Sullys Hill National Game Preserve is located on the lake's southern shore. Grahams Island State Park is located on an island in the lake. Other parks on the lake include Black Tiger State Recreation Area and Shelvers Grove State Recreation Area, which is now closed due to the lake's flooding.
A proposal from the Army Corps of Engineers would have drawn water from a different point of the lake, included filtration, and been able to discharge a maximum of 300 ft³/s (8 m³/s) of water from Devil's Lake, though it would be subject to the full battery of federal bureaucracy and international negotiation.
The North Dakota government under Governor John Hoeven instead pursued construction of an outlet, with approval of the North Dakota Department of Health and the U.S. State Department and Council For Environmental Quality. With limited state funding this outlet has a lower maximum discharge than the federal proposal (limited to a maximum of 100 ft³/s (2.8 m³/s) by the Section 402 NDPDES Permit) and with a gravel filter, to remove larger organisms, but is faster in providing some relief to the situation. Construction began in 2003 and was completed as of summer 2005.
The outlet has drawn opposition from the governments of Minnesota and Manitoba, which maintain that the outlet would create the potential for unacceptable levels of sulfates in the river, and the potential for the transfer of unknown foreign aquatic species into the Red River basin (including Lake Winnipeg, the world's 10th-largest freshwater lake), as the outlet would release untreated water from the lake.
In March 2004, Manitoba, along with Minnesota and several environmental groups, sued the Department of Health in state court over the Devils Lake Outlet 402 NDPDES Permit. The outlet was allowed to proceed in state court rulings August 2004 and May 2005, however, federal courts and the International Joint Commission have yet to weigh in on the issue.
The Canadian government has taken a particular interest in the matter, arguing that completion of the outlet as currently planned would be a violation of the Boundary Waters Treaty of 1909 and Canada's national sovereignty. The U.S. has taken the position that the diversion will not be harmful or violate the treaty under current conditions.
In November 2005, a joint United States and Canadian study concluded that none of the 13 species Canada classifies as invasive were present in Devil's Lake. However, the study did find three species of fish parasites that are not currently known to exist in Lake Winnipeg. However, all three are ubiquitous to the waters of North America and have a wide variety of hosts. A difficulty inherent in determining what species are in Devils Lake versus Lake Winnipeg results from spatial relationships. Devils Lake is approximately 193 mi² (502 km²) in area. Lake Winnipeg covers approximately 9,400 mi² (24,300 km²), or almost 49 times larger than Devils Lake. In addition, the Devils Lake Basin is significantly smaller than the watersheds feeding Lake Winnipeg, including the Saskatchewan River basin at approximately 56,000 mi² (1,450,000 km²). Drawing conclusions about the biological community already in Lake Winnipeg is difficult, due to the relative lack of biological sampling compared to Devils Lake. To that end, joint efforts by the United States and Canada began a biannual biological survey of Devils Lake, the Sheyenne River, the Red River, Lake Traverse, and Lake Winnipeg beginning in 2006, and ending in 2008. The data from the first year of sampling indicates that the fish in Devils Lake are healthier than even the fish in other waterbodies in the region. Highlighting the lack of knowledge about biological communities outside of Devils Lake, two alien parasite species were found in Lake Winnipeg, the Asian tapeworm, Bothriocephalus acheilognathi and the blood dwelling trematode, Sanguincola occidentalis. To date, the Government of Manitoba has not swayed from its position that the State of North Dakota install a more extensive filtration system at the outlet of the lake than is already in place.
A 2006 poll by Friends of the Earth Canada found that the majority of people in Manitoba and North Dakota were aware of the issue and "67.3% of North Dakotans agreed with the statement, 'As a good neighbor, North Dakota should not allow polluted water to enter Canada.' This statement was supported by nearly all of the Manitobans (96%)".
Some preventative measures have been proposed, including inserting chemicals in the creeks along the boundary of the Devil's Lake Basin and the Red River Valley that do not allow fish to live. Biologists did tests in 2005 which conclude that there are currently no carp in the Devil's Lake Basin, but some have been found within two miles (3 km). The carp appear to have been stalled by the abundant cattail plants, which makes travel impossible for the fish.