Precious water flows from vertically-tilted limestone at Big Spring.
Sinking streams usually are symptomatic of karst, “a landscape formed from the dissolution of soluble rocks including limestone, dolomite and gypsum ... characterized by sinkholes, caves, and underground drainage systems” (source). Rainwater is slightly acidic, enough to dissolve rock such as limestone. If it can enter soluble bedrock via fractures or faults, dissolution will create a network of enlarging passageways, accommodating more water, leading to more dissolution and surface collapse.
|"What is Karst?" – from the University of Texas at Austin.|
Water also exits the karst underworld, as springs. An underground drainage system may intersect the surface, perhaps at a fault, or meet fractures in overlying rock that provide routes to the surface.
|Modified from Karst in Indiana; Indiana Geological Survey.|
In the Uinta Mountains in northeastern Utah, limestone bedrock, outcrops and karst features occur on both flanks of the range.
“Another peculiarity of topographic structure is observed at the head of the side ravines ... when they occur in limestone formations. In the midst of a remarkably well watered region, these ravines have no running water, and in their basin-like heads are many minor depressions without outlet” Samuel F. Emmons, Uinta Mountains (1906)Uinta streams head in the high country, which is underlain by Precambrian sandstones of the Uinta Mountain Group. They flow above ground until they meet the Madison limestone, where many of the smaller ones disappear, joining underground drainages that feed springs at lower elevations.
North flank Uinta Mountains west of Flaming Gorge. Darker turquoise unit is the Madison limestone.
Cross-section through north flank Uinta Mountains at Sheep Creek Canyon. Near the Uinta Mountain fault zone, strata are tilted to vertical or even overhanging. Yu - Uinta Mountain Group; Mm – Madison limestone. Modified from Sprinkel et al. (2000).
Spectacularly deformed strata in the Sheep Creek Canyon Geological Area. Mississippian Madison limestone on right and Precambrian Uinta Mountain Group on left; southwest branch Uinta Mountain fault zone in-between.
Madison limestone (Mississippian).
Water first appears near the base of a small talus slope – just a tiny quiet leafy pool.
In less than ten feet, it becomes a healthy flowing stream.
Discharge from Big Spring ranges from 5 to 36 cubic feet per second. Like most springs in the area, its flow is variable and highest in spring during snowmelt. Turbidity increases too, with higher flows stirring up sediments in the underground drainage system.
Years ago, Big Spring’s flow suddenly increased and became more turbid ... but it was the wrong time of year. Just a week before, an earthen dam 14 miles to the west had failed, sending precious water into the Lost Creek Sink a short distance downstream. Ranchers were unhappy. Hydrologists were suspicious.
Sometimes underground drainage routes can be inferred from coincident events – like a breached dam and an unexpected pulse in a spring. They can be verified with dye tracers. In 1979 researchers dumped dye into Lost Creek Sink and watched Big Spring. Dye emerged, confirming the connection. Another dye release at low water in September 2001 also showed a link between sink and spring, with maximum groundwater travel time of 14 days (Spangler 2005).
Lost Creek Sink is 14 miles west of Big Spring; both are within the north-dipping Madison limestone. Other sinks and springs in the northeastern Uinta Mountains show similar configurations, suggesting underground drainages follow strike (along limestone layers rather than across them). In contrast, springs on the less-steep south side of the range tend to be artesian, flowing from the Madison up through fractures in overlying rocks to the surface (Spangler 2005).
The map below shows general direction of flow from sink to spring (the actual route probably is far more tortuous). Straight-line distance is 14.4 miles, with 2100 feet vertical drop. As of 2005, Lost Creek - Big Spring was “one of the longest documented (dye-traced) underground flow systems in Utah.” The length suggests there may be an extensive cave system below (Spangler 2005).
|Based on Spangler 2005; map from ArcGIS Online.|
“Hence the explanation that suggested itself was that, in the easily soluble limestones, surface waters had eaten their way along cracks and small faults, finding their run-off in such springs, and had thus eroded increasingly large caves that had finally collapsed, producing something analogous to the sink-holes of the western Appalachian region. ... The structure is developed on so large a scale in this region that it deserves a special name, for which sink-hole or karst topography is suggested.” Samuel F. Emmons, Uinta Mountains (1906)
Emmons, SF. 1906. Uinta Mountains. Bulletin of the Geological Society of America 18:287-302.
Spangler, LE. 2005. Geology and karst hydrology of the eastern Uinta Mountains – an overview. in Dehler, CM, Pederson, JL, Sprinkel, DA, and Kowallis, BJ, eds. 2005. Uinta Mountain geology. Utah Geological Association Publication 33.
Sprinkel, DA, Park, B and Stevens, M. 2000. Geologic road guide to Sheep Creek Canyon Geological Area, northeastern Utah, in Anderson, PB and Sprinkel, DA, eds., Geologic road, trail and lake guides to Utah's parks and monuments. UGA Publ 29. PDF