Longest undammed river in the contiguous United States AI-pedia

The question of which river is the longest undammed river in the contiguous United States depends on the technical definition of a dam. By the standard geographical definition—a river lacking large, water-storing reservoirs or hydroelectric dams across its main stem—the Yellowstone River holds the title. Flowing 692 miles (1,114 km) from its headwaters in northwestern Wyoming through Montana to its confluence with the Missouri River in western North Dakota, the Yellowstone moves across high mountain plateaus and the northern Great Plains without a single mainstem flood-control or hydroelectric storage impoundment.

A second river, the Salmon River in central Idaho, is also frequently cited as the longest undammed river in the contiguous United States. Flowing 425 miles (685 km) entirely within Idaho before emptying into the Snake River, the Salmon River has no storage dams and no low-head diversion structures on its main channel. While the Yellowstone is 267 miles longer than the Salmon, the Yellowstone is crossed by several low-head agricultural diversion weirs. The Salmon River is therefore the longest river in the contiguous United States with an entirely unobstructed channel, as well as the longest undammed river contained within a single state.

Both rivers represent rare survivals of pre-industrial North American hydrology. In the contiguous United States, more than 90,000 dams listed in the National Inventory of Dams have altered the flows, sediment cycles, and temperatures of almost every major river system. The Yellowstone and the Salmon maintain natural flow regimes—seasonal patterns of rising and falling water driven by melting snowpack rather than the operation of mechanical spillways. This hydrological condition supports unique ecosystems, sustains endangered native fish species, and provides scientists with baseline models for how healthy, self-regulating river systems function.

Defining Undammed and Free-Flowing Waterways

In hydrologic and civil engineering, water-control structures are classified by their purpose, height, and effect on streamflow. The distinction between a storage dam and a diversion weir is central to the debate over which river is the longest undammed waterway in the lower 48 states.

A storage dam is a tall structure built across a river valley to impound a large volume of water. Storage dams create artificial lakes or reservoirs behind their walls. These reservoirs hold back water during high-flow seasons and release it gradually during dry seasons for hydroelectric power generation, municipal supplies, navigation control, or flood protection. By retaining water over months or years, storage dams eliminate the natural seasonal flood pulse, alter water temperature profiles, trap riverbed sediments, and permanently submerge terrestrial ecosystems.

A diversion weir, or low-head dam, is a much smaller structure built across a river to raise the upstream water level by a few feet. It does not impound a vast reservoir or store seasonal runoff. Instead, it maintains a consistent water elevation so that a portion of the river's flow can be diverted by gravity into side canals for agricultural irrigation or municipal use. The excess water simply flows over the crest of the weir and continues downstream. Because low-head weirs do not store floodwaters, the river downstream retains its natural hydrograph—a chart showing changes in discharge over time.

Under federal agency guidelines and general hydrologic definitions, the Yellowstone is classified as free-flowing and undammed because its main stem has no storage reservoirs that control its seasonal hydrograph. The Salmon River, conversely, meets the stricter ecological definition of an unobstructed river, possessing neither storage dams nor low-head diversion weirs along its entire main stem.

The Yellowstone River System

The Yellowstone River drains an expansive basin of approximately 70,000 square miles (180,000 km²), spanning northern Wyoming, southern and eastern Montana, and the western fringe of North Dakota. It is the principal tributary of the upper Missouri River, delivering an average discharge of 13,773 cubic feet per second (390 m³/s) at its lower monitoring station near Sidney, Montana.

Course and Geography

The river begins at the Continental Divide in the Absaroka Range of northwestern Wyoming, formed by the meeting of its North and South Forks at an elevation above 12,000 feet (3,650 m). From this wilderness origin, the river flows north into Yellowstone National Park, entering Yellowstone Lake—the largest high-elevation lake in North America above 7,000 feet. Leaving the northern end of the lake at Fishing Bridge, the river traverses the Hayden Valley before dropping over the Upper Falls (109 feet) and the Lower Falls (308 feet) into the Grand Canyon of the Yellowstone.

Continuing northward through the Black Canyon, the Yellowstone leaves the park near Gardiner, Montana, and enters Paradise Valley, a broad structural basin framed by the Absaroka Range to the east and the Gallatin Range to the west. At Livingston, Montana, the river bends abruptly eastward, turning away from the Rocky Mountains to begin its long journey across the northern Great Plains.

Across Montana, the river passes through Billings, Miles City, and Glendive. The surrounding landscape transitions from pine-covered sandstone bluffs to open, semi-arid plains. Near Buford, North Dakota, just east of the state boundary, the Yellowstone empties into the Missouri River. At this confluence, the Yellowstone is often larger by volume than the Missouri River itself, contributing the primary share of cold, sediment-rich water to the upper Missouri channel.

Hydrology and Tributaries

The Yellowstone River is fed by major mountain and plains tributaries. Its left-bank tributaries, which join from the north and west, include the Gardner River and the Shields River. Its right-bank tributaries, entering from the south and east, drain large portions of the Absaroka, Beartooth, and Bighorn mountain ranges. These include:

• Lamar River: Drains the northeastern corner of Yellowstone National Park and delivers high sediment loads during mountain snowmelts.
• Boulder River: Originates in the Absaroka-Beartooth Wilderness and enters near Big Timber, Montana.
• Stillwater River: Originates near the Beartooth Plateau and provides significant cold-water inflows near Columbus, Montana.
• Bighorn River: The largest tributary of the Yellowstone, entering near Custer, Montana. The Bighorn is heavily regulated by Yellowtail Dam, making it the primary contributor of regulated, clear-water flows to the lower Yellowstone.
• Tongue River: Rises in the Bighorn Mountains of Wyoming and passes through southeastern Montana, joining at Miles City.
• Powder River: Originates in central Wyoming and enters near Terry, Montana. The Powder River is itself completely undammed on its main stem, contributing vast quantities of natural sediment that give the lower Yellowstone its characteristic turbidity.

Because the main stem of the Yellowstone is not regulated by storage dams, its discharge varies dramatically throughout the year. At the Sidney, Montana stream gauge, minimum recorded winter flows drop to roughly 3,100 cubic feet per second (88 m³/s), while peak spring runoff driven by mountain snowmelt has reached 159,000 cubic feet per second (4,500 m³/s).

Water Infrastructure on the Yellowstone River

Although the Yellowstone River is free of large storage reservoirs, its waters have been heavily utilized for agricultural production since the late 19th century. Six major low-head diversion weirs cross the main channel between Columbus and Glendive, Montana.

Structure Name	Location / River Mile	Year Constructed	Primary Purpose	Structural Type
Waco-Custer Diversion Dam	Near Custer, MT (RM 360)	1907	Agricultural irrigation	Rock and timber-crib weir
Huntley Diversion Dam	Near Huntley, MT (RM 352)	1934 (rebuilt)	Huntley Project irrigation	Concrete overflow weir
Cartersville Diversion Dam	Forsyth, MT (RM 237)	1932	Agricultural irrigation	Concrete and rock weir
Myers Diversion Dam	Near Hysham, MT (RM 312)	1919	Irrigation canal intake	Rock-fill weir
Intake Diversion Dam	Near Glendive, MT (RM 71)	1905–1911	Lower Yellowstone Project	Rock-filled timber/concrete weir

The most prominent of these structures is the Intake Diversion Dam, located roughly 71 miles upstream from the river's mouth. Constructed by the United States Bureau of Reclamation as part of the Lower Yellowstone Project, Intake is a 12-foot-high (3.7 m) weir that diverts water into a main canal serving over 50,000 acres of farmland in eastern Montana and western North Dakota.

For more than a century, Intake Diversion Dam served as an impassable barrier to several native fish species, most notably the pallid sturgeon (Scaphirhynchus albus). Pallid sturgeon require long stretches of free-flowing, turbid river to complete their reproductive cycle. Following decades of biological research and environmental litigation, the Bureau of Reclamation and the U.S. Army Corps of Engineers completed a 2.1-mile (3.4 km) concrete-free bypass channel around Intake in 2022. The bypass allows sturgeon and other migratory species to swim upstream past the weir for the first time since 1910, opening access to approximately 165 miles of historical spawning habitat in the Yellowstone and Powder rivers.

The Salmon River: The Contender for Absolute Undammed Status

The Salmon River of Idaho is the longest river in the contiguous United States that contains no artificial barriers of any kind on its main stem—neither large storage dams nor low-head diversion weirs. Draining a mountainous, sparsely populated watershed of roughly 14,000 square miles (36,000 km²), it runs entirely within Idaho's borders for 425 miles (685 km).

Course and Wilderness Character

The Salmon River originates near Galena Summit in the Sawtooth Range, at an elevation of more than 8,700 feet (2,650 m). It drops rapidly through the Sawtooth Valley, flowing past the towns of Stanley, Clayton, and Challis. At the town of Salmon, it turns west and cuts directly through the northern Rocky Mountains.

This central section forms a canyon deeper than the Grand Canyon of the Colorado River. Much of its path traverses the Frank Church-River of No Return Wilderness, the largest contiguous wilderness area in the lower 48 states, spanning 2.36 million acres. Because of the sheer granite cliffs and roadless terrain, early European explorers and river runners named it the "River of No Return"—wooden boats could navigate downstream through its powerful rapids, but they could not return upstream against the current.

After emerging from the wilderness canyon near Riggins, the Salmon River turns north, then west, forming the geographical boundary between Idaho's Mountain and Pacific time zones. It joins the Snake River along the Oregon-Idaho border in the depths of Hells Canyon. At the White Bird gauging station, the river discharges an average of 11,060 cubic feet per second (313 m³/s).

Ecological Role and Anadromous Fish Migrations

Because the main stem of the Salmon River is completely unobstructed, it provides vital spawning habitat for anadromous fish—species that are born in freshwater, spend most of their adult lives in the ocean, and return to freshwater to spawn. Species using the Salmon River corridor include:

• Chinook Salmon (Oncorhynchus tshawytscha): Spring, summer, and fall runs migrate up the river to spawn in high-elevation gravel beds.
• Sockeye Salmon (Oncorhynchus nerka): Travel to Redfish Lake and Little Redfish Lake in the Sawtooth Valley. This journey covers 900 miles (1,450 km) from the Pacific Ocean and requires ascending 6,500 vertical feet (2,000 m), representing the longest and highest elevation sockeye migration in North America.
• Steelhead Trout (Oncorhynchus mykiss): Sea-run rainbow trout that make extensive use of the main stem and its tributaries, such as the Middle Fork and South Fork of the Salmon.
• Bull Trout (Salvelinus confluentus): A native char listed as threatened under the Endangered Species Act, which requires very cold, pristine headwater streams.

While the mainstem Salmon River has no dams, its anadromous fish must still navigate eight large hydroelectric and navigation dams built on the lower Snake River and the Columbia River to reach the ocean and return. The loss of juvenile fish in downstream turbines and slackwater reservoirs, along with adult mortality during upstream migration, has led to severe declines in the Salmon River's wild fish runs since the 1960s.

Comparison: Yellowstone River vs. Salmon River

The following table summarizes the key physical, geographical, and infrastructural differences between the two rivers most frequently identified as the longest undammed waterways in the contiguous United States.

Characteristic	Yellowstone River	Salmon River
Total Length	692 miles (1,114 km)	425 miles (685 km)
Watershed Area	~70,000 sq mi (180,000 km²)	~14,000 sq mi (36,000 km²)
Average Discharge	13,773 cfs (390 m³/s)	11,060 cfs (313 m³/s)
States Crossed	Wyoming, Montana, North Dakota	Idaho (intrastate)
Mouth	Missouri River (near Buford, ND)	Snake River (Hells Canyon)
Storage Reservoirs	Zero on main stem	Zero on main stem
Low-Head Diversion Weirs	6 major agricultural weirs	Zero on main stem
Dominant Fish Assemblage	Potamodromous (sturgeon, paddlefish, cutthroat trout)	Anadromous (Chinook, sockeye, steelhead)
Primary Geographical Feature	Northern Rocky Mountains to Great Plains	Deep wilderness canyons and alpine valleys
Longest Undammed Category	Longest free-flowing (no storage dams)	Longest completely unobstructed (no weirs/dams)

Ecology of Undammed Rivers

Undammed rivers function under the principles of the natural flow regime. When water is not held behind storage dams, the physical shape of the river channel, the surrounding plant life, and native animal cycles are driven by natural variations in weather and climate.

Morphology and Channel Dynamics

A natural river is dynamic; its channel continuously moves across its valley. During annual spring floods, high volumes of water carry sand, gravel, and large boulders downstream—a process called sediment transport. The energy of these floods cuts into outside riverbanks and deposits gravel on inside bends, forming point bars and mid-channel islands.

This continuous movement creates a braided or meandering channel pattern with complex habitats, including:

• Riffles: Shallow, fast-moving sections where water flows over clean gravel, providing oxygen-rich habitat for aquatic insects and fish spawning.
• Deep Pools: Slower-moving, deep pockets of water carved out near river bends and fallen logs, offering shelter and cooler temperatures for adult fish during summer heat.
• Side Channels and Sloughs: Quiet backwaters connected to the main channel that serve as nurseries for larval fish and feeding grounds for waterfowl.
• Gravel Bars: Exposed banks of freshly deposited stone where pioneer plant species can take root.

On dammed rivers, storage reservoirs trap sediment, releasing clear water downstream. This sediment-starved water erodes the riverbed, digging a deep, narrow trench (incision) and washing away gravel bars without replacing them. On the Yellowstone and the Salmon, the natural supply of sediment from mountain headwaters remains intact, preserving complex aquatic habitats.

Riparian Forest Recruitment

The health of riparian forests—the zones of trees and shrubs growing directly along riverbanks—is closely linked to the natural flood cycle. Plains cottonwood (Populus deltoides) and narrowleaf cottonwood (Populus angustifolia) dominate the floodplains of the Yellowstone River.

Cottonwood seeds are light and fluffy, dispersing through the air in late spring. To germinate and survive, these seeds require bare, moist, sunlit silt beds that have been recently cleared by floodwaters. Furthermore, the seedling roots must grow downward at the same rate that the water table drops as spring floods recede. On dammed rivers where floods are suppressed, cottonwood forests stop regenerating and gradually die out. Along the Yellowstone, seasonal flood pulses continue to create the conditions necessary for forest regeneration.

Thermal and Chemical Regimes

Storage dams dramatically alter downstream water temperatures. Deep reservoirs release either freezing water from their depths during summer or unnaturally warm water during winter. These artificial temperature regimes disrupt the life cycles of aquatic organisms, which rely on seasonal temperature cues to spawn, hatch, or emerge from the water.

In contrast, undammed rivers maintain their natural thermal cycles. In winter, surface ice forms extensively along the Yellowstone and upper Salmon rivers. In summer, water temperatures rise in response to direct sunlight and ambient air temperatures. Native species are evolutionarily adapted to these wide temperature swings.

Historical Threats: Dam Proposals on the Yellowstone and Salmon

Neither the Yellowstone nor the Salmon River remained free-flowing by accident. Both systems were targeted repeatedly by federal agencies, utility companies, and agricultural interests for major dam projects throughout the 20th century.

The Allenspur Dam and Yellowstone Lake Proposals

The first major threats to the Yellowstone River targeted its source inside Yellowstone National Park. In the 1920s and 1930s, agricultural cartels in Montana and Idaho lobbied the United States Congress to build a storage dam at the outlet of Yellowstone Lake. The proposed dam would have raised the lake level by several feet to store water for downstream irrigation. Strong opposition from the National Park Service, conservation organizations, and national figures defeated these bills, preserving the park's natural waters.

Further downstream, the United States Army Corps of Engineers and the Bureau of Reclamation identified a prime dam site in Paradise Valley, south of Livingston, Montana. Known as the Allenspur Dam site, the proposal called for a massive concrete structure across a natural narrowing of the valley. Had it been built, the resulting reservoir would have flooded roughly 30 miles of Paradise Valley, submerging working ranches, destroying vital trout habitat, and backing water up toward the town of Gardiner and the northern border of Yellowstone National Park.

The Allenspur proposal resurfaced several times between the 1930s and the 1970s, promoted alternately for irrigation storage, flood control, and coal-development energy production during the 1970s oil crisis. A coalition of local ranchers, fly-fishing guides, conservation groups, and Montana politicians organized fierce opposition. They argued that the economic value of agricultural lands and recreational tourism far outweighed the benefits of an impoundment. The Allenspur Dam was permanently abandoned in the late 1970s.

The Nez Perce and High Mountain Sheep Proposals

During the mid-20th century, the Pacific Northwest was the center of massive hydroelectric dam construction managed by the Army Corps of Engineers and private power companies. After projects on the Columbia and lower Snake rivers were authorized, engineers turned their attention to the confluence of the Snake and Salmon rivers.

Two rival dam proposals emerged in the 1950s:

• Nez Perce Dam: A proposed massive concrete dam located on the Snake River just below the mouth of the Salmon River. Nez Perce would have backed water up both the Snake and Salmon rivers, completely blocking all salmon and steelhead migration into the entire Salmon River drainage basin.
• High Mountain Sheep Dam: A proposed dam on the Snake River located just above the confluence with the Salmon River.

Fisheries biologists argued that building the Nez Perce Dam would entirely destroy Idaho's remaining wild salmon runs. After intense debate before the Federal Power Commission and in the federal courts, the High Mountain Sheep site was selected instead, leaving the mouth of the Salmon River unobstructed. Later, the passage of the Central Idaho Wilderness Act of 1980 and protections under the National Wild and Scenic Rivers Act permanently protected the main stem of the Salmon River from hydroelectric development.

Legal and Administrative Protections

Undammed rivers in the United States are preserved through a patchwork of state water compacts, federal wilderness protections, and public access laws.

The Yellowstone River Compact of 1950

Water rights on the Yellowstone River and its tributaries are governed by an interstate agreement among Wyoming, Montana, and North Dakota, ratified by Congress in 1951. The Yellowstone River Compact allocates the unappropriated waters of four major tributaries: the Clarks Fork, Bighorn, Tongue, and Powder rivers. While the compact focuses on water allocation for agriculture and industry rather than environmental preservation, it recognizes pre-existing water rights and sets limits on out-of-basin diversions, preventing catastrophic dewatering of the main stem.

Montana Stream Access Law

The Yellowstone River is heavily protected by Montana's Public Trust Doctrine and its landmark 1985 Stream Access Law. Under Montana law, the public may use any river or stream capable of recreational use for activities like fishing, floating, and swimming up to the ordinary high-water mark, regardless of who owns the adjacent dry land. This legal framework has fostered an active recreational and angling community that serves as a political barrier against attempts to dam or heavily divert the river.

The National Wild and Scenic Rivers Act

Passed in 1968, the National Wild and Scenic Rivers Act protects selected rivers with outstanding natural, cultural, and recreational values from being dammed or altered by federal projects. In 1980, large sections of the Salmon River and its primary tributary, the Middle Fork of the Salmon, were officially designated under the Act.

Designated segments are classified into three management categories:

• Wild River Areas: Rivers free of impoundments and generally inaccessible except by trail, with primitive shorelines and unpolluted waters. The Frank Church canyon of the Salmon River falls under this classification.
• Scenic River Areas: Rivers free of impoundments, with shorelines largely primitive and undeveloped, but accessible in places by roads.
• Recreational River Areas: Rivers that are readily accessible by road or railroad, that may have some shoreline development, and that may have undergone past impoundment or diversion.

While portions of the Yellowstone within Yellowstone National Park are protected by federal park regulations, the main stem of the Yellowstone through Montana is not designated under the federal Wild and Scenic Rivers Act, relying instead on state environmental laws and water leasing programs to maintain minimum instream flows.

Environmental Pressures and Contemporary Management

Despite lacking large storage dams, undammed rivers face growing ecological and infrastructural pressures in the 21st century.

Climate Change and Hydrographic Shifts

Undammed rivers depend entirely on mountain snowpack to feed their spring and summer discharge. Warmer winter temperatures lead to more precipitation falling as rain rather than snow, resulting in smaller winter snowpacks. As a result, the spring snowmelt peak occurs several weeks earlier in the year than it did historically.

By late summer, when ambient temperatures are highest, water levels drop to critical lows. Lower flows warm more rapidly, reducing dissolved oxygen levels and stressing cold-water fish like trout and salmon. In August 2016, an unprecedented outbreak of proliferative kidney disease (caused by the parasite Tetracapsuloides bryosalmonae) killed tens of thousands of mountain whitefish on the upper Yellowstone River, prompting Montana Fish, Wildlife & Parks to temporarily close 183 miles of the river to all public recreation to stop the parasite's spread.

Catastrophic Flooding: The June 2022 Yellowstone Flood

In mid-June 2022, an atmospheric river brought warm, heavy rainfall to the Absaroka and Beartooth ranges just as the high-elevation snowpack was reaching its spring melt. The combination of torrential rain and rapid snowmelt produced a historic 500-year flood event on the upper Yellowstone River.

Peak discharge at the Corwin Springs gauge reached over 54,000 cubic feet per second, shattering the previous record set in 1918. The raging waters washed out entire sections of the highway between Gardiner and Livingston, destroyed bridges, undermined home foundations, and caused extensive channel avulsions—sudden shifts in where the main riverbed flows. Because there were no storage dams to absorb the floodwaters, the river expanded across its natural historical floodplain, demonstrating both the destructive power and the geomorphic vitality of an unmanaged river system.

Industrial and Energy Corridors

The lower Yellowstone River valley serves as a transportation and pipeline corridor for energy extraction in the Williston Basin (Bakken formation) and the Powder River Basin. The river is crossed by multiple crude oil pipelines buried beneath its bed.

On July 1, 2011, ExxonMobil's Silvertip Pipeline—a 12-inch crude oil pipeline crossing the bed of the Yellowstone near Laurel, Montana—ruptured during high spring floodwaters. The pipeline had become exposed by riverbed scouring, spilling an estimated 1,500 barrels (63,000 gallons) of crude oil into the river. The oil slick drifted more than 80 miles downstream, coating gravel bars, backwaters, and agricultural fields. A second major pipeline rupture occurred near Glendive in January 2015 when the Bridger Pipeline breached beneath the river ice, spilling approximately 30,000 gallons of oil and contaminating the municipal drinking water supply of Glendive.

Other Long Undammed Reaches in the Contiguous United States

Aside from the Yellowstone and Salmon rivers, several other river segments in the lower 48 states preserve long, contiguous stretches of free-flowing water. In most cases, these are free-flowing reaches embedded within overall systems that are bounded by upstream or downstream storage dams.

River System / Segment	Length of Free-Flowing Stretch	Location	Primary Limiting Factors
Yellowstone River	692 miles (1,114 km)	Wyoming, Montana, North Dakota	Full mainstem length; bounded at mouth by Missouri River; has low-head weirs.
Salmon River	425 miles (685 km)	Idaho	Full mainstem length; zero dams or weirs; mouth joins dam-regulated Snake River.
Upper Missouri River (head of Fort Peck)	~207 miles (333 km)	Montana	Upper segment from Great Falls/Morony Dam to Fort Peck Lake reservoir.
Green River (WY/UT)	~385 miles (620 km)	Wyoming, Colorado, Utah	Long continuous segment between Flaming Gorge Dam and the Colorado River confluence.
Owyhee River System	~340 miles (547 km)	Nevada, Idaho, Oregon	Extensive tributary network; lower reach terminated by Lake Owyhee reservoir.
Delaware River	330 miles (530 km)	New York, Pennsylvania, New Jersey, Delaware	Longest undammed river in the eastern United States; no dams from source to Delaware Bay.
Atchafalaya River	137 miles (220 km)	Louisiana	Free-flowing distributary, though origin is controlled by the Old River Control Structure.

In contrast to the contiguous United States, the state of Alaska contains numerous large rivers that remain completely undammed from source to sea. The Yukon River, which flows for 1,980 miles (3,190 km) across Canada and Alaska, has small storage facilities on headwater lakes in Canada, but its 1,200-mile main stem across Alaska is entirely undammed. The Kuskokwim River (702 miles) and the Tanana River (584 miles) in Alaska are both longer than the Yellowstone and Salmon, and are completely free of dams along their entire channels. However, within the geographical limits of the lower 48 states, the Yellowstone and Salmon remain the benchmark standards for large-scale, undammed river systems.

See also

• Fluvial geomorphology
• Frank Church-River of No Return Wilderness
• List of longest undammed rivers
• National Wild and Scenic Rivers Act
• Natural flow regime
• Pallid sturgeon
• Riparian zone
• Salmon River (Idaho)
• Yellowstone National Park
• Yellowstone River