Hydrogeomorphology and Springs
Geomorphology is the study of landforms, so hydrogeomorphology is basically the study of landforms where groundwater has played a critical role in sculpting the land. On Earth the role of water is evident, particularly when Earth is compared to the heavily cratered, very ancient surfaces of the other terrestrial planets and most moons. However, even on Earth the critical role of groundwater in geomorphic processes is underappreciated, as it is conventionally judged to be inferior to effects produced by overland flow. Here again, dogma has been substituted for fundamental observation.
Figure 1. Alluvial fan in Iran provides recharge of groundwater used down gradient for farming. Photo USGS
The dogma is familiar enough. Fluvial systems are dominated by dendritic drainage systems, where steep, v-shaped valleys in upland areas change downstream into braided streams and ultimately to meandering rivers in broad valleys, accompanied by a progressive reduction in topographic slope (see Figure 5 of Hydrological Cycle chapter). While not entirely meaningless, this viewpoint ignores the fact that most of the water in rivers and streams got there via subsurface pathways; otherwise our rivers would become nearly dry every time a week passes without significant rain.
The processes essential to hydrogeomorphology are conveniently divided into recharge and discharge. Most emphasis will be on the latter, as emerging groundwater is more evident, and the former has already been considered in a separate chapter.
Areas of groundwater recharge are those where groundwater is flowing downhead and into the ground. Such areas are common along hilltops, highlands above the hinge line, and also in relatively flat areas where lateral surface runoff is inefficient. Downward infiltration can be very efficient in areas underlain by fractured basaltic rocks, or in volcanic plateaus, flat prairies, karst uplands, and in the clastic alluvial aprons bordering mountains (Figure 1). In many plateau and flat upland areas, virtually all meteoric precipitation infiltrates downward, so surface streams and dendritic networks are almost completely absent. Vast regions of Earth have a character indicating that downward infiltration and recharge, not lateral surface runoff, is the dominant process.
Karst uplands provide an excellent example of vertical infiltration, and this condition is widespread because carbonate rocks underlie about 17% of Earth’s land area. Karst areas are dominated by subsurface drainage and associated solutional landforms. An estimated 25% of Earth’s population relies on the shallow groundwater systems that lie immediately below. Many karst regions comprise sinkhole plains that are relatively flat areas pockmarked by as many as several hundred sinkholes per square kilometer (Figure 2). These regions are nearly devoid of surface drainages, yet caves with flowing streams abound in the shallow subsurface. Interestingly, many cave systems embody branching networks, somewhat reminiscent of dendritic surface streams, but cave systems are less developed and constituted of a few large channels. Lateral inflows to these main trunk passages are commonly inconspicuous, probably being dominated by flows along numerous fractures, joints and bedding planes. These inflows originate from sinkholes that most commonly lie laterally away from the main cave streams, rather than directly above them. In contrast, collapsed cave ceilings tend to be more elongate than circular, unlike typical sinkholes.
Figure 2. Sinkholes in Florida define the landscape and lead to groundwater recharge. Picture from USGS.
Other areas of groundwater recharge include losing reaches of streams, as well as reservoirs where the hydraulic head has been artificially increased by impoundment. Of course, the latter condition can be problematical as the dam itself can be undermined by subsurface flows (Figure 3).
Figure 3. Teton Dam failure that starts out with groundwater undermining the earthen dam and eventually leads to failure. Pictures from Wikipedia. Photo by Mrs. Eunice Olson, 5 June 1976.
Discharge areas are groundwater “outcrops”. Springs are the most familiar of these, but also included are gaining streams, seeps, fens, oases, and most wetlands, ponds, mountain meadows, and natural lakes. Areas where emerging groundwater may be important yet less conspicuous or intermittent include prairies, rock shelters, and areas of saline soils or quicksand. In dry areas, zones of emerging groundwater commonly feature a vegetated fringe with a more saline, playa-like interior.
Flow in gaining streams increases downstream. This augmentation is mostly due to groundwater contributions, commonly by inconspicuous inflows below the stream surface, rather than by obvious inputs by entering tributaries.
Seeps and many wetlands and ponds are primarily fed by groundwater inflows. Such areas, as well as the banks of gaining streams, occur where the water table intersects Earth’s surface. Seeps and many wetlands are basically springs where groundwater emerges, but in a distributed rather than a concentrated manner.
Fens are commonly but erroneously called bogs. Typical bogs are acidic and organic-rich, and most commonly occur in low areas where surface runoff accumulates. In contrast, fens are areas of groundwater emergence, and their waters tend to be alkaline.
Springs also are points where the water table intersects Earth’s surface, and groundwater emerges at a single localized orifice. Springs are of many types. Springs may be perennial or ephemeral; fresh or saline; or cool or hot. In contrast to cool freshwater springs, the flows of thermal and saline springs are driven by density or pressure differences that are generated by buoyancy or compressional forces.
Types of Springs
The flows of most springs are driven by hydraulic heads that are controlled by normal hydrologic forces. Nevertheless, the geologic environments where springs emerge are very diverse.
Karst springs are the most common type, and probably include the largest discharges of groundwater from a single orifice. These springs are the distal zones where karst groundwaters, commonly recharged in sinkhole plains, reemerge at lower elevations, commonly after flowing through tens or hundreds of kilometers of cave systems. Perhaps the most widely known are the huge springs that emerge near the coast of west-central Florida. Florida’s springs are recharged by the heavy rainfall that occurs in the central part of the state, which then passes through young sandy deposits to enter the subjacent Floridan aquifer. The groundwater then flows to the west and southwest, ultimately to emerge in several huge coastal springs. Note that despite its size and heavy rainfall, Florida has no large rivers.
The Ozark region of Missouri and Arkansas also hosts many large karst springs. This region is primarily underlain by karstified Paleozoic dolostones and limestones, although the karst surfaces are commonly mantled by thick residuum that masks the expression of sinkholes and other features that would normally be obvious. Subsurface recharge is very common in this area and feeds the huge Ozark aquifer. Many broad valleys in the Ozarks have no flowing streams, except during brief intervals following intense storms, because the water enters the subsurface so quickly. This region hosts many “first magnitude” springs that have flows of 3 to 15 cms. Dye traces of the largest spring, appropriately called Big Spring, show that some of its recharge originates as much as 30 km from its orifice, and that the groundwater basin contributing Big Spring’s flow covers more than 1000 km2 (Figure 4).
Figure 4. Big Spring, Missouri is a “first magnitude” karst spring with an average flow of 15 cms. It discharges from a collapsed cave in the Cambrian Eminence dolomite, and drains a >1000 km2 area underlain by the Ozark aquifer.
Contact Springs emerge along the interfaces of geologic formations that have greatly differing permeabilities. Most common are springs in sedimentary sequences where sandstone overlies shale. Infiltrating waters enter the sandstone, flowing laterally and downward until the impermeable shale is encountered. Then the flow must move laterally until the host formation breaches the topographic surface and the water emerges, typically along a reentrant in a canyon wall. Similar permeability contrasts commonly occur in volcanic piles, where impermeable flow units are intercalated with permeable “aa” flows and intercalated horizons of soils or lake deposits, altogether constituting multiple hydrogeologic contrasts (Figure 5). In an extreme example, the Lost River of Idaho completely disappears where it sinks into the complex, volcanic-sedimentary pile beneath the Snake River Plain. The Lost River spectacularly reemerges about 200 km to the west at Thousand Springs, situated in the walls of the entrenched Snake River Canyon. A negative heat flow anomaly shows the trace of the lost underground “river”.
Figure 5. Burney Falls in northern California illustrates how groundwater emerges from contacts with different volcanic layers on the left side of the photo. Photo by MLD.
Another type of contact spring exists where steep vertical dikes cross stream valleys. Groundwater is forced to emerge when the impermeable dikes are encountered.
Faults can likewise cause permeability contrasts that are associated with groundwater emergence, either because rocks of contrasting properties are juxtaposed, or because impermeable, fine grained gouge formed along the fault itself. Faults can sometimes be recognized by linear belts of vegetation where groundwater emerges, particularly in arid areas such as occur along the Banning fault of southern California. In detail the consequences of faulting are complex. Faults can either serve as conduits because of the fractures themselves and their associated breccias, or they can serve as groundwater barriers due to the formation of gouge, or because impermeable rocks are juxtaposed. Many hydrothermal ore deposits occur along faults, clearly indicating enhanced permeability. However, conditions along faults can be dynamic, with faulting and brecciation being followed by mineral deposition, cementation, and complete permeability loss.
Depression Springs can occur where the topographic surface, for whatever reason, intersects the water table. Springs and sometimes ponds and lakes can occur in these areas. For example, small localized springs are common along roadcuts, but these are generally intermittent.
Joint and Fracture Springs can occur in bedrock that would otherwise have low permeability. Such springs occur, for example, in the crystalline rocks of the Sierras and the Piedmont. Flows are typically small, but the groundwater in fractured rocks can sometimes supply residences. Giant Spring of the Great Falls Montana is a prominent example of a fracture control spring discharge (Figure 6).
Figure 6. Giant Spring in Great Falls Montana emerges from joints and liniments associated with the regional sandstone layer. Outflow discharges into the Missouri River. Photo by MLD.
Quaternary Volcanic Springs Many recent volcanic deposits (less than 1 Ma), particular those of basaltic composition host long-range subsurface conduits that were previously lava tubes (Figure 7). Once the lava cools an empty cavern remains many miles long (Figure 8). These caverns tend to fill with groundwater and at the end of the conduit high volume spring discharge will emerge. The volcanic edifice is the source region of the basaltic flow and acts capture precipitation that infiltrates and recharges these buried conduits. In the Pacific Northwest of the US there is voluminous basalts known as the Columbia River Basalt Group, which is many thousands of feet thick. These basalts host numerous subterranean conduits that travel for many miles as illustrated for the Lost River discussed earlier and give rise to a number of first magnitude spring discharges. Hawaii host many subterranean aquifers in the basaltic rock where long-range flow persists. Some Hawaiian freshwater springs emerge in the ocean, while others emerge on high flanks of the volcanic edifice, while others transport downhill to emerge at low topography.
Figure 7. Big Springs the McCloud River in northern California discharges groundwater about 600 cfs from the toe of a recent basalt flow. Photo by Caltrout.
Figure 8. Subway Cave in Hat Creek Valley California illustrates the subsurface conduits left behind by long-range basalt flows. Photo by MLD.
In northern California there is a number of first magnitude springs. These springs emerge out of the ends of basalt flows. In one case Mt Lassen which last erupted in the early 1900s and recharges melting snow pack and that groundwater travels 45 kilometers before emerging out of the end of a the Hat Creek basalt flow (Figure 9). The groundwater stable isotope data is identical to snowmelt on the flanks of Mt Lassen.
Figure 9. Rising River emerges at the terminus of a 45 km long flow path from Mt Lassen through subsurface basalt. Note the basalt in the bottom right corner of the photo. Photo by MLD.
Further north there is Medicine Lake Volcano, which gave rise to abundant long-range basalt flows. At the end of these basalt flows emerges 34 cubic meters per second of freshwater discharge, literally a river coming out of the ground. The stable isotope composition of the spring flow is identical to snow melt on the Medicine Lake Volcano, and the infiltrating groundwater travel 60 kilometers to emerge out the springs. The age dates of these spring discharges are less than 30 years old (Figure 10).
Figure 10. Literally a river emerges at the terminus of a 60 km flow path from Medicine Lake Volcano and creates the Fall River at an average of 1200 cfs. Photo by MLD.
Thermal Springs are common in areas of recent magmatism, typically less than 1 Ma, but they can also occur where very deep circulation of water is promoted by great topographic relief or other circumstances. Most hot springs in subaerial areas represent meteoric waters that become heated by hot, proximal magmas, then are driven by their buoyancy. The waters move toward the hot magmas, become heated and less dense by thermal expansion, then they ascend, being pushed laterally inward and then upward by the flow of denser, cooler meteoric waters at distance. During this path the fluids become geochemically evolved, because they interact with hot host rocks. Where strongly heated the fluids tend to become saturated in silica and evolve into Na-Ca-Cl waters, which are relatively dilute and have near neutral pH (Figure 11). The overall process is driven by the remarkable properties of water near its critical point, which for pure water is about 374°C. Under critical conditions, the thermal expansivity, density and therefore the buoyancy of the fluid is maximized, and its heat capacity is maximized, just as its viscosity is minimized. Thus, the capability of the fluid to freely move and transport heat is maximized as the critical point is approached. As the hot fluid ascends, however, it must boil and cool, so it eventually emerges at the surface under near-boiling conditions (Figure 11).
Low pH fluids also emerge in many areas of recent volcanism (Figure 11). These “acid sulfate” springs have hydrogen ion as the dominant cation, and sulfate as the dominant anion, so they are basically springs of sulfuric acid. These springs may form by subsurface evaporation above a geothermal system dominated by the hot, neutral pH chloride-type waters described above, or by the heating of meteoric waters by ascending, sulfuric, volcanic gases that they absorb. In any case, stable isotope studies prove that these waters are dominantly of meteoric origin.
Figure 11. Left. Morgan Hot Springs is a neutral pH, chloride spring south of Lassen Volcanic National Park. The temperature of 89°C is boiling at its altitude. On rapid cooling these fluids precipitate white siliceous sinter. Right. Bumpas Hell is a fumarole that discharges superheated (159°C), acid sulfate steam with a pH of 4.4. The corrosive fluids have altered the andesitic host rocks to a mixture of cristobalite and kaolinite. Pyrite is being actively precipitated from the pH 2.2, blue-white pool near the boardwalk. Photos by REC.
Somewhat analogously, ocean water in submarine hydrothermal systems becomes heated and buoyant near mid-ocean ridges, and eventually rise to emerge on the sea floor as spectacular “black smokers”. Rapid mineral deposition occurs where these rising, near critical fluids encounter sea water that is just above freezing. Many important copper and zinc sulfide deposits appear to have formed in this way.
Discharge of Springs
Spring discharge is more steady than that of proximal surface streams, because overland flow contributions are minimized and the subsurface residence time of the water is generally longer. For this reason, springs were preferred sites for pioneer mills, particularly where large flows issued from orifices well above the flood-prone valley bottoms. Flow variations nevertheless occur, particularly in cave streams following intense storms, and sometimes prove fatal for spelunkers (Figure 12).
Figure 12. Stage and discharge hydrographs for Big Spring, Missouri during 2017. While the average discharge for 2017 was above-average at 518.7 cfs, flow varied from a minimum of 342 cfs to 1340 cfs during the large flood that impacted the Ozarks in April. Note that large storms superimpose discharge pulses on a regular annual hydrograph pattern. Data from USGS.
The long-term average flows of springs behave similarly to those of surface streams in the same region. In temperate regions, when the average flows of streams and springs are plotted against basin area on a log-log plot, a trend with a unit slope is defined (see Figure 14 of the Surface Water chapter). Of course, the size of a groundwater catchment is more difficult to define than the area of a surface watershed whose boundaries are topographic (see below). Springs whose average flows exceed 100 cfs (2.8 m3/s) are called “first magnitude” springs; those with 10 to 100 cfs (0.28 to 2.8 m3/s) are termed “second magnitude” springs; those with 1 to 10 cfs (0.028 to 0.28 m3/s) are called “third magnitude” springs, etc. There are only 69 first magnitude springs in the USA, more than half of which drain karst aquifers in Florida and the Ozarks.
Geochemical Variations of Springs
The geochemistry of spring water is highly varied, as the mineral matter is derived from the host rock of the aquifer, which vary in character. The mineral matter is introduced by water-rock interactions that also depend on the spring temperature and the length of time available for the reactions to occur.
The most dilute springs are in cold, mountain regions where water, largely derived from snowmelt, enters fractured igneous rock, particularly insoluble and unreactive granitic rocks. If the residence time is short, these springs can discharge water that is little more saline than rainfall. If heated by igneous intrusions, however, water-rock interactions can significantly increase the mineral content of meteoric-hydrothermal waters, with diverse outcomes (see Figure 11, above).
Among the clastic aquifers, springs associated with sandstones are likely most pure, while those associated with intercalated shales can have high and undesirable contents of iron and sulfate.
Karst groundwaters are very common and widely used. Encounters with the host limestones or dolostones impart a Ca-Mg-HCO3 chemistry to the water. Because flows through cave systems can be rather rapid, shallow karst groundwaters and associated springs are easily contaminated by agrichemicals and bacteria. In most areas these waters are definitely not safe to drink, without boiling or filtration.
Variations in spring discharge are highly correlated with variations in the temperature, turbidity, and chemistry of spring water. In general, the more variable the flow, the more variable are other parameters. Turbidity can significantly increase with discharge, along with the concentrations of ions that are associated with or adsorbed on soil particles. The concentrations of most dissolved ions decrease with discharge, however. The variations in chemical concentrations resemble discharge hydrographs, but are commonly inverted, and have different time constants than that for flow (see Winston and Criss, 2004). Those time constants can differ parameter to parameter, so that when the concentration of a particular ion is plotted against another, a common result is a hysteresis loop, rather than a straight line that would be expected if the process were simple dilution.
Pseudokarst involves the development of karst-like features in clastic sediments, by erosional rather than solutional processes. The time scale for the development of these features is generally short, sometimes involving a time frame of a single large storm. Pseudokarst features are common in drylands and badlands, where subsurface erosion can be the major geomorphic process.
The principal pseudokarst processes are called piping and sapping. Piping involves the formation of tubular drainage passages by water percolating through clastic materials, with the pipes commonly developing along shrinkage cracks in clay material. Piping is commonly related to the temporary saturation of soils above the water table (Figure 13).
Figure 13. Left. Cave developed in siltstone by piping, Anza Borrego State Park, CA. Photo by EM Criss. Right. Rock shelter in sandstone, produced by sapping, Shawnee National Forest, IL. Photo by REC.
Sapping is the process of seepage-related undermining, followed by collapse of overlying rocks, forming amphitheater-headed valleys. Sapping can cause largescale scarp recession, and its action is often concentrated at valley heads. Sapping produces theater-headed, U-shaped valleys by headward erosion at seepage faces where groundwater, and commonly springs emerge. Gullies, cliff caves, overhangs and rock shelters, and natural bridges and arches are produced by sapping. Mass wasting and landslide processes are also largely due to sapping, and groundwater plays an essential role (Figure 14). On a small scale, cavernous weathering can produce an “alveolar”, or honeycomb, texture on steep rock surfaces. Mass wasting, and landslide processes are associated of sapping.
Figure 14. Small mudslide developed after heavy rainfall in Jan. 2005, above Cal State San Bernardino. Note the theater-headed shape of the scarp, and the association of the slide with groundwater. Photo by EB Melchiorre.
Pseudokarst processes are best developed in dry areas, such as the USA southwest, especially in the Colorado Plateau. They also appear to be common on Mars. The stubby, steep headed valleys in these regions strongly suggest that emerging groundwater is essential to this process (Figure 15).
Figure 15. Satellite photo of part of the Grand Canyon, showing the regional development of a network of stubby, theater-headed valleys by pseudokarst processes. Photo credit: NASA.