The San Juan River is a major tributary of the Colorado River in the Southwestern United States, providing the chief drainage for the Four Corners region of Colorado, New Mexico, Utah, and Arizona. Originating as snowmelt in the San Juan Mountains (part of the Rocky Mountains) of Colorado, it flows U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed March 21, 2011 through the deserts of northern New Mexico and southeastern Utah to join the Colorado River at Glen Canyon.
The river drains a high, arid region of the Colorado Plateau. Along its length, it is often the only significant source of fresh water for many miles. The San Juan is also one of the muddiest rivers in North America, carrying an average of 25 million US tons (22.6 million t) of silt and sediment each year.
Historically, the San Juan formed the border between the territory of the Navajo in the south and the Ute in the north. Although Europeans explored the Four Corners region as early as the 1700s, it was not settled until the gold and silver booms of the 1860s, when settlers arrived in large numbers from the eastern United States. After heated conflicts over land, the Native Americans were forced into reservations, where their descendants live today.
During the 20th century, intensive drilling in the fossil-fuel rich San Juan Basin and uranium mining along the lower river in Utah generated serious concerns about water quality, particularly in the Navajo Nation where the river is a crucial source of water for irrigation. Runoff from abandoned gold and silver mines has also been a major issue, as occurred in the 2015 Gold King Mine waste water spill into the Animas River, the main tributary of the San Juan.
The U.S. federal government has built a number of large dams in the San Juan River system to control floods and to provide irrigation and domestic water supply. In addition, the lower part of the river is inundated by Lake Powell, one of the largest reservoirs in the United States. Efficient management is crucial to ensuring enough water supply not just for farms and urban areas but also for recreational boating, fisheries, and environmental restoration. However, heavy water use has significantly reduced the flow of the San Juan River by as much as 25 percent since pre-development conditions. In addition, warming temperatures in the Rocky Mountains are projected to have a further negative effect on snowpack and thus stream flow.
Course
The San Juan River begins in Archuleta County, Colorado at the confluence of its East and West Forks. Both forks originate above elevations of in the eastern San Juan Mountains in the San Juan National Forest. The river flows southwest through the foothills of the Rocky Mountains through the town of Pagosa Springs and reaches the Navajo Lake reservoir just north of the New Mexico border, near Arboles, Colorado. Below the Navajo Dam, the San Juan River flows west through a narrow farming valley in the high desert country of the Colorado Plateau. At Farmington, New Mexico, it is joined from the north by its main tributary, the Animas River, which rises in the San Juan Mountains near Silverton, Colorado.
From there it flows west through the Navajo Nation, turning northwest near Shiprock and its namesake monolith, crossing very briefly back into southwest Colorado (within of the Four Corners quadripoint) before entering southeastern Utah. West of Bluff, Utah the river slices through the Comb Ridge and enters a series of rugged winding canyons, often over in depth. The lower of the San Juan River, in a remote portion of the Glen Canyon National Recreation Area, are flooded by Lake Powell, which is formed by Glen Canyon Dam on the Colorado River. The San Juan joins the Colorado in San Juan County, Utah at a point about to the north of Navajo Mountain and northeast of Page, Arizona.
Tributaries of the upper San Juan River above Navajo Dam include the Rio Blanco and the Navajo River, and the Piedra River and Los Pinos River (Pine River) which join the San Juan in Navajo Lake. In addition to the Animas River, several major tributaries join below Farmington, including the La Plata River and Mancos River in New Mexico, and McElmo Creek in Utah. San Juan also has a number of seasonal tributaries that drain arid regions of the Colorado Plateau. These include the Cañon Largo and Chaco River in New Mexico and the Montezuma Creek and Chinle Creek in Utah. The northern tributaries of the San Juan River, which originate in the San Juan Mountains, are snowmelt-driven, with the highest flows between March and June. Southern tributaries such as the Chaco River are mostly ephemeral but can carry large volumes of water during flash floods.
Discharge According to the U.S. Bureau of Reclamation, the average unimpaired runoff or natural flow of the San Juan River basin over the 1906–2014 period was about , per year. The maximum was , , in 1941, and the minimum was , , in 2002. Heavy water use has decreased the river flow considerably since the early 20th century. The U.S. Geological Survey (USGS) stream gaging station at Bluff, Utah, recorded an average annual discharge of , or , for the 1915–2013 period. Before the construction of major dams to regulate the river, it sometimes dried up completely in the summer, as it did in 1934 and 1939. The maximum flow was on September 10, 1927. The great flood of October 1911, which remains the largest recorded flood on the San Juan River, occurred before the USGS began measuring streamflow here. Based on observations of water depth and debris deposits, the 1911 flood may have reached a peak of .
USGS real-time stations on the San Juan RiverGage location(ID number)StartyearAveragedischargeMaximumdischargeMinimumdischargeDrainageareaPercent oftotalwatershedPagosa Springs, CO093425001935 ft3/s(10 m3/s) ft3/s(708 m3/s) ft3/s(0.2 m3/s) mi2(728 km2)1.1%Carracas, CO093464001961 ft3/s(17 m3/s) ft3/s(243 m3/s) ft3/s(0.02 m3/s) mi2(3,240 km2)5.1%Archuleta, NM093555001954 ft3/s(30 m3/s) ft3/s(535 m3/s) ft3/s(0.2 m3/s) mi2(8,443 km2)13.3%Farmington, NM093650001931 ft3/s(56 m3/s) ft3/s(849 m3/s) ft3/s(0.8 m3/s) mi2(18,750 km2)29.4%Shiprock, NM093680001935 ft3/s(56 m3/s) ft3/s(2,265 m3/s) ft3/s(0.2 m3/s) mi2(33,400 km2)52.4%Four Corners, CO093710101977 ft3/s(55 m3/s) ft3/s(479 m3/s) ft3/s(3 m3/s) mi2(37,800 km2)59.3%Bluff, UT093795001914 ft3/s(61 m3/s) ft3/s(1,982 m3/s) ft3/s(0 m3/s) mi2(59,700 km2)93.5%Stations are listed from furthest upstream to furthest downstream. Although river discharge is generally expected to increase from upstream to downstream, note that discrepancies in the data may be due to differences in the period of record, as well as human modifications, diversions, and dams. The San Juan River annual hydrograph exhibits large seasonal variations with the highest monthly flow of in June, and the lowest of in December. The completion of Navajo Dam in 1963 and its associated water supply projects have decreased the flow of the river, from about in the 1914–1963 period to less than for the 1964–2016 period, although a minimum release from the dam prevents the river from drying up in the summer. Persistent drought conditions in the 21st century have further reduced the flow of the San Juan River, with an average annual discharge of or between water years 2000 and 2016.
Geology The oldest geologic feature of the San Juan River basin is the San Juan Mountains, which consist largely of Precambrian and Paleozoic crystalline rock and outcrops of Tertiary volcanic rock. During the Paleocene and Eocene (about 66–34 million years ago), streams draining off the southern flank of the San Juan Mountains terminated in the San Juan Basin around present-day Farmington, slowly filling it with thick layers of sediment. Over millions of years, the burial of organic material under sedimentary layers created the abundant coal, oil, and natural gas deposits found in the area today.
Fluvial deposits indicate that a stream flowed west across the Colorado Plateau to join the Colorado River at least by the late Miocene (about 5 million years ago). This may have been the ancestral Dolores River, before the uplift of the Ute Mountains diverted it to its present northerly course. When the San Juan Basin filled and overflowed, it formed an outflow channel west into the old Dolores River bed, establishing the San Juan River's modern course.
Though Tectonic forces about 2–3 million years ago caused the terrain to rise across the Monument Upwarp in southeast Utah and northeast Arizona, the river maintained its course as an antecedent stream. The Monument Upwarp consists of a series of parallel anticlines and synclines running generally north to south in an area roughly long and wide. Where the river passes through these formations, it has sliced deep canyons through the reddish rock. In places, the San Juan has entrenched its ancient meanders thousands of feet into the bedrock, as can be observed in Goosenecks State Park, where it winds through a set of horseshoe bends while traveling a straight-line distance of only . The canyon cutting was accelerated during the Pleistocene Ice Ages when the climate of the area was much wetter. The wetter climate resulted in floods of up to –ten times larger than any flooding of the San Juan in recorded human history.
The San Juan River flows through highly erodible sedimentary rock–such as sandstone, siltstone, and shale–that make up rock formations such as the slide-prone Chinle Formation. As a result, the San Juan is an extremely muddy river, contributing more than half of the sediment that occurs in the upper Colorado River above Lees Ferry despite accounting for only 14 percent of the total runoff.
Between 1914 and 1980, the U.S. Geological Survey station at Bluff, Utah, recorded an average of 25.41 million tons of sediment carried by the San Juan River each year—ranging from 3.24 million tons in 1978 to 112.4 million tons in 1941. The majority of the sediment originates in the watershed downstream of the Animas River.
Since the closure of the Glen Canyon Dam in 1963, sediments have been building a delta in Lake Powell instead of continuing down the Colorado River. Core samples taken in 2011 indicate aggradations up to thick in the San Juan River arm of Lake Powell.
Watershed Geography
The San Juan River drains approximately in Colorado, New Mexico, Utah, and Arizona; the watershed land area is almost the same size as the state of West Virginia. The main stem of the river does not flow through Arizona but comes very close at the Four Corners. The highest point in the San Juan River watershed is Windom Peak, located near the headwaters of the Animas River. The lowest elevation, where the San Juan River flows into Lake Powell, has a normal maximum elevation of but typically fluctuates dozens of feet per year due to seasonal nature of runoff in the Colorado River Basin affecting the reservoir level. Most of the watershed is rural, with some extremely remote, uninhabited areas. About 75 percent of the watershed is shrubland, rangeland, and grassland. Due to the low rainfall and lack of substantial groundwater reserves, agricultural use is scarce, except for river valleys and in higher foothill areas influenced by wetter montane climates. About 2.3 percent of the watershed is dryland farms, and 1.6 percent is irrigated. Forests cover about 20 percent of the watershed, mostly at high elevations.