A Groundwater Sapping in Stream Piracy

Home , Groundwater sapping, Headward erosion, Stream capture, Stream competency

Darryll T. Pederson, Department of energy to the system as increased logic settings, such as in a delta, stream Geosciences, University of Nebraska, recharge causes groundwater levels to piracy is a cyclic event. The final act of Lincoln, NE 68588-0340, USA rise, accelerating stream piracy. stream piracy is likely a rapid event that should be reflected as such in the geo- INTRODUCTION logic record. Understanding the mecha- The term stream piracy brings to mind nisms for stream piracy can lead to bet- ABSTRACT an action of forcible taking, leaving the ter understanding of the geologic record. Stream piracy describes a water-diver- helpless and plundered river poorer for Recognition that stream piracy has sion event during which water from one the experience—a takeoff on stories of occurred in the past is commonly based stream is captured by another stream the pirates of old. In an ironic sense, on observations such as barbed tribu- with a lower base level. Its past occur- two schools of thought are claiming vil- taries, dry valleys, beheaded streams, rence is recognized by unusual patterns lain status. Lane (1899) thought the term and elbows of capture. A marked of drainage, changes in accumulating too violent and sudden, and he used change of composition of accumulating sediment, and cyclic patterns of sediment “stream capture” to describe a ground- sediment in deltas, sedimentary basins, deposition. Stream piracy has been re- water-sapping–driven event, which he terraces, and/or biotic distributions also ported on all time and size scales, but its envisioned to be less dramatic and to be may signify upstream piracy (Bishop, mechanisms are controversial. Some the common mechanism for stream 1995; Pissart et al., 1997; Mather et al., researchers conclude that stream piracy piracy. Crosby (1937) took issue with 2000; Johnsson, 1999). Recognition that is a rare event and happens only on Lane (1899 and later papers) and argued piracy is happening now is based on small scales; this is based on a recogni- that surface water is the principal agent observed higher erosion rates for streams tion that surface-water energy decreases of stream piracy in most settings. This with steeper gradients on one side of a near divides and the belief that ground- set up a debate on the relative roles of drainage divide relative to the other, water-sapping processes decrease in surface-water erosion and groundwater- with the steeper gradient stream captur- effectiveness near divides and are not sapping erosion that persists today. This ing the headwaters of the lower gradient effective in rock and cohesive sediment. paper contends that groundwater-flow stream (Bates, 1961; Vogt, 1991; Ries et In contrast, numerous studies show that patterns and groundwater-sapping pro- al., 1998). As variants, development of groundwater-sapping is effective in rock cesses are important in most cases of karst aquifers can lead to underground and cohesive sediment, focused by the stream piracy, and the final act of piracy capture of rivers over time, such as the intersection of the extending channel can be rapid because of the developing recognition of loss of upper Danube with the water table, and effective in geometry. There is a predictable imprint flows (Hötzl, 1996), and cyclic develop- hillslope processes. Further, destruction of groundwater flow, and groundwater ment of lobes on the Holocene Missis- of evidence by surface water is the rea- sapping is effective at all scales and in sippi River delta is modulated by stream son for the general lack of recognition all geologic material. capture (Roberts, 1997). of groundwater-sapping effects. I argue The issue of stream piracy is more While the identity of the villain (mech- that the persistence of groundwater-flow than an academic discussion because it anism) remains controversial, there are systems, coupled with the evolving geo- is an important geologic process—past common elements in stream piracy metry as a pirating stream approaches a and present. Sediment-deposition pat- regardless of the erosion process. To divide, can sustain breaching by ground- terns and mineralogy can be drastically have energy to do the work of erosion water-sapping processes. The principal altered with the event of stream piracy. and transport, the pirating stream needs determinant of the maintenance of Stream piracy can change migration pat- to be at a lower elevation or have a energy is the position of the groundwa- terns for aquatic animals and can change steeper gradient. In addition, the geologic ter divide as compared to the topo- rates of erosion in upland areas. Stream material near where the capture takes graphic divide where streams in adjacent chemistry can be changed as a conse- place must be susceptible to disaggrega- drainage basins are at different eleva- quence of stream piracy. In some geo- tion by mechanical or chemical processes tions. Wetter climatic periods can add

4 SEPTEMBER 2001, GSA TODAY or to solution, or must already be disag- gregated. The process of erosion at the channel head coupled with the geologic setting and energy in the system deter- mines the rate and direction of channel extension. There must be a mechanism for transport of eroded sediment from the channel head. Bishop (1995) argued that in most settings, there is minimal energy for stream piracy and therefore little piracy. This paper will show that the evolving groundwater-flow system as a pirating stream approaches the divide can provide the threshold energy for breaching.

GROUNDWATER AND STREAM PIRACY The phenomenon of groundwater “sapping” has been identified with a number of terms. Higgins (1984) expanded the term “seepage erosion” to encom- Figure 1. Erosion by needle ice growth in a road bank, Boone, North Carolina, USA. pass the more complex erosion of con- Groundwater extrusion represented by ice columns several cm long. solidated rock. He included intensified chemical weathering; leaching and dissolution within the seepage zone; and Lawler (1993) reported a similar result Groundwater Sapping Examined enhanced physical weathering by granu- from needle-ice growth on the River The effect of positive pore-water pres- lar disintegration or flaking owing to Ilston, South Wales, United Kingdom. sure in promoting erosion and increasing wetting-drying, salt-crystal wedging, root Prosser et al. (2000) found that needle- instabilities of slopes is well known. An wedging, rainbeat, congelifraction, and ice growth in winter and desiccation of expansion of the concept is needed to re- needle-ice wedging. Howard (1988a) fur- clays in the summer control erosion on late it to stream piracy. In computer ther expanded the overall concept Ripple Creek Canal, Tasmania. Surface- modeling and laboratory studies (Howard, proposing “Groundwater sapping, as water flows were unable to erode firm, 1988b), headcuts spontaneously formed distinct from piping, is a generic term for cohesive clay banks that had not been in response to groundwater seepage, weathering and erosion of soils and rock preconditioned by groundwater sapping. and they migrated up gradient because by emerging groundwater, at least par- I have found that needle-ice growth is of their intersection with the groundwa- tially involving intergranular flow (as responsible for road-bank erosion (Fig. 1) ter table where positive pore pressures opposed to the channelized throughflow and erosion of lakeshore banks (Fig. 2). occur, promoting erosion. With continued involved in piping)” (p. 3). Groundwater sapping is used in this context in this paper. The study of groundwater-sapping processes and their extension to stream piracy is complicated by the presence of surface water, which in many cases destroys evidence of groundwater sapping. For example, I have observed freezing groundwater in a vertical river- bank near Cook, Nebraska, that led to the dislodging of frozen bank material in meter-sized blocks during January 1999. Spring freeze-thaw cycles fragmented the blocks into transportable sediment that was carried away by a June flood. A July visitor would recognize significant bank erosion since the previous summer visit, but, because all evidence for groundwater sapping was destroyed, would attribute this to the June flood. Figure 2. Erosion by groundwater sapping (one week of freeze-thaw cycles) on shoreline bank of Lake Ashtabula, North Dakota, USA. With melting of lake ice, wave action removed the talus.

GSA TODAY, SEPTEMBER 2001 5 headward development, groundwater water table parallels the beach face, gul- variety of groundwater-sapping processes flow was focused in the few uppermost lies advance updip without increasing and the nearly universal presence of headcuts that interfered minimally with incision. By extension, to divide areas, if groundwater, it is likely that most hillslope each other. These continued to erode the water-table slope is less than the sur- erosion and channel extension patterns headward, intercepting the groundwater face slope, increasing entrenchment carry the imprint (pattern) of groundwa- flow that would have gone to the now- would be favored where groundwater ter-flow systems. Freeze (1987) modeled inactive headcuts. As groundwater-dis- sapping is the dominant erosion process the resulting water table between parallel charge energy in the form of positive at the headcut. The development of a rivers with given conditions of recharge pore pressures is rapidly focused in the drainage network in the Finisterre and aquifer characteristics and coupled few remaining headcuts, a consequent Mountains in New Guinea (Hovius et al., the output to slope-stability calculations acceleration of headward erosion takes 1998) may be similar to Higgins’ beach to show effects of recharge from various place, and a roughly parallel drainage example. Initial gorge incision is precipitation events. His results showed network is developed. Dunne (1998) expanded by large-scale landsliding con- that hydrogeologic factors coupled with described a similar positive feedback in trolled by groundwater seeping, with the climatic variation and short-term precipi- hollow formation. On a smaller ground- resulting debris being incised by fluvial tation events exerted considerable con- water-flow scale, Collison’s (1996) simu- erosion. Montgomery and Dietrich trol on the slope of the surface topog- lations showed that even small soil cracks (1988) reported that channel initiation raphy between the rivers in terms of (enhancing recharge) just upstream from on steep slopes in the Coos Bay region stability of slopes and channel banks as gully heads resulted in positive pore of Oregon and in the southern Sierra controlled by pore pressures. His results pressures at the base of the headcut. suggest that the surface topography can Collison (1996) also found that most be a reflection of the groundwater table eroded material transported by surface as opposed to the popular concept of the water originates in the gully itself rather water table being a subdued reflection than upslope, so the controlling factor In broader terms,because of the of the surface topography. This sugges- for gully expansion should be headcut tion is supported by the consequences and wall instability. Flume experiments great variety of groundwater-sapping of hillslope processes and the nature of (Kochel et al., 1988; Baker et al., 1990) channel extension in divide areas. that modeled headcut migration under processes and the nearly universal More specifically, groundwater sapping hydrogeologic conditions found in the presence of groundwater,it is likely can be recognized by distinctive patterns Colorado Plateau yielded long valleys, of erosion occurring as a consequence short tributaries, and amphitheater that most hillslope erosion of the geologic setting and regional and heads, comparing well with the field local groundwater-flow systems. Fyodor- description of Laity and Malin (1985). and channel extension patterns ova and Sasowsky (1999) described the The ability of a river to erode, trans- dissolution of quartz cement by ground- port, and deposit sediment is in part carry the imprint (pattern) of water leading to increased porosity and determined by interaction with the reduction of the mechanical strength of groundwater system. As an example, groundwater-flow systems. a sandstone. This weakening has led to positive pore-water pressures in areas additional groundwater-sapping processes where groundwater discharges into a becoming active with development of stream can increase the erodibility of the caves along preexisting fractures and stream bed. Changes of erodibility and Nevada is associated with landsliding joints within the sandstone bedrock. transport competency with groundwater that is probably caused by seepage ero- They suggest that dissolution of silica is influx and outflux in beach and stream sion. In most settings, the actual sapping transport controlled so the process is settings can favor sediment accumula- rates should be a function of groundwa- most active where fractures have been tion or enhance erosion (Harrison and ter-flow rates. Gabbard et al. (1998) enlarged and groundwater has not Clayton, 1970; Howard, 1988b; Butt and found in laboratory studies that intro- adjusted chemically. (The bigger fractures Russell, 2000). duction of groundwater inflow caused with greater flow continue to grow.) Several case examples show the accelerated headward erosion with ero- Norris and Back (1990) described a simi- nature of the interaction of the ground- sion rates increasing by 60 times. The lar evolution where mixing of groundwater-flow system and headcut migra- importance of groundwater-sapping ero- water and seawater leads to dissolution tion in field settings of possible incipient sion in geologic processes should not be of carbonate rock along the Yucatan. It stream piracy. Higgins (1984) observed overlooked (Higgins, 1984; Dunne, 1980; should be noted that groundwater is that when the water table slopes at an Schumm, 1980; Roloff et al., 1981; Netto nearly as effective and rapid in erosion angle less than that of the beach face, et al., 1988). Dunne (1990) rated water of siliceous rocks as of limestone rocks gully heads incise deeper as they as second only to gravity in producing in terms of chemical weathering and advance updip. There is a threshold in slope instabilities. removal of dissolved substances (Young gully depth where small slumps and and Saunders, 1986). Johnsson (1999) block slides, due to deep entrenchment, The Imprint of Groundwater Flow identified two sets (based on orientation negate further advancement. Where the In broader terms, because of the great and elevation) of horizontal cave passages

6 SEPTEMBER 2001, GSA TODAY associated with different historical eleva- extending channel results in a migration tions of the water table and directions of of the groundwater divide toward the groundwater flow in the karst of Swago soon-to-be-pirated stream. More of the Creek in West Virginia. In each case, the regional recharge to the groundwater- fracture pattern of the rock and the flow system is now moving toward the direction of the groundwater gradient extending channel in stage B (Fig. 3) were contributing factors to the pattern with a likely increase in groundwater- of groundwater sapping. Nash (1996) sapping potential. At some point in time, found that the headward development the soon-to-be-pirated stream will start of valleys in the Hackness Hills in North losing flow to the groundwater system, Yorkshire, England, was in part con- which will further increase the ground- trolled by groundwater-sapping processes water-sapping potential. At this point, operating in an updip direction. the potential exists for the initiation of If groundwater-sapping location is a sediment accumulation in the pirated function of groundwater-flow systems channel. In contrast, the energy for and headcut interception, then ground- headcut erosion by surface water is water-flow models should suggest pat- decreasing because of the decreasing terns of development of stream drain- Figure 3. Cross-section sequence (A–C) of catchment size. ages. In fact, drainage patterns based on pirating channel extension and eventual In stage C (Fig. 3), the pirated stream groundwater-flow models using field divide breaching by groundwater sapping. is losing considerable flow to the ground- hydrogeologic parameters and recharge Arrows show paths of groundwater flow and water system and the potential for sedi- rates show close agreement with actual inverted triangle shows position of water ment accumulation is high. Breaching of field settings. Streams of a given order table. Sedimentation in pirated stream shown the topographic divide was possible (in the sandy Pleistocene area of the by higher density of dots. Erosion in pirating because the groundwater-flow system is Netherlands) can be explained as out- channel shown by lower density of dots. able to maintain its energy and, in fact, crops of groundwater-flow systems of a may experience an increase in energy corresponding order reflecting the spond to the surface-water divide when gradient as the pirated stream is drainage density necessary to effectively there is a difference in elevation of approached by the extending channel drain the aquifer system (DeVries, 1976, streams in the adjacent drainages (Fig. 3). of the pirating stream. An additional 1994). DeVries (1995) expanded his Because of this, the groundwater-flow increase in energy in the groundwater- work to include a model of contracting system does not lose its energy with flow system will occur with the accumu- and expanding stream networks with gully extension like the surface-water lation of sediment in the pirated stream. groundwater-level change, as related to system does with its decreasing catch- A positive-feedback situation develops seasonal rainfall characteristics. Troch et ment size. Three selected stages of chan- in which losses of water from the pirated al. (1995) applied DeVries’s (1976) model nel extension across a divide are shown stream augment the groundwater-sap- to the Zwalmbeek catchment in Belgium. in Figure 3. ping process, leading to further sediment Coupled with the observation that in In stage A, a tributary of the pirating accumulation and other outcomes. many humid lowland areas, overland stream is extending itself by headward While the basic process shown in the flow is rare and so most flow is under- erosion toward the divide. Groundwater model is the same in heterogeneous and ground, they submitted that the existing flow is focused, much like flow to a anisotropic settings, the actual flow paths drainage network developed through pumping well. The energy driving the followed by groundwater in these set- sapping erosion at the zone of ground- groundwater-flow system is reflected by tings may modify the geometric pattern water exfiltration. In a setting compara- the difference between the elevation of shown in Figure 3. Also, developing ble to that described by DeVries, the the groundwater divide and the eleva- fracture flow or development of karst drainage network in the Sand Hills of tion of the water-table outcrop at the (limestone or rock with soluble cement) Nebraska consists of roughly parallel headcut. The energy of the surface-water may cause stream piracy to occur well rivers with no tributaries. The drainage- system would be the difference between before there is an apparent surface network density corresponds to the thick- the topographic divide and the base of expression. It is only a matter of time ness of the underlying aquifer; it is less the headcut. Note, no mass considera- before the subsurface flow paths would dense where the aquifer is thicker and tions or energy conversions that occur be expressed at the surface. was likely developed by headward ero- along the flow paths are included, so It is very difficult to identify areas of sion caused by groundwater sapping this is not a measurement of the actual incipient stream piracy on a large scale (Pederson, 1995). energy available at the headcut for ero- because the process is slow compared sion. The elevation and location of the to the human time scale, and the The Groundwater-Sapping Model of groundwater divide represents a dynamic quantification of channel extension rates Stream Piracy equilibrium that will change with cli- is complicated by the nonlinearity of the The principal fact favoring stream matic changes and changing geometries erosion system. It is also very difficult piracy by groundwater sapping is that such as channel extension. to determine past rates of stream piracy the groundwater divide does not corre- Continued headward erosion of the because the evidence has usually been

GSA TODAY, SEPTEMBER 2001 7 destroyed. Having said that, the proposed model of stream piracy by groundwater sapping should work on all time and size scales as the mechanics should operate at all scales.

SURFACE WATER AND STREAM PIRACY The basics of a “surface water view” of stream capture (the other potential villain) were described by Crosby (1937) and are still found in many textbooks. Erosion is “gnawing back at the headwaters of every stream” (p. 469) with the rate of erosion dependent on the forma- tions present, slope of the land, climatic conditions, and protecting vegetation. Weathering breaks rocks into fragments, which are transported by hillslope processes—including sheetwash, landslides, and soil creep—to stream channels where they are transported. Rates of erosion are controlled by water velocity, Figure 4. Rainbow Falls, near Hilo, Hawaii, USA. There is a clear undercutting of the falls’ face abrasion tools, and the underlying for- well beyond the zone of plunge. The presence of adjacent springs and water flowing from the mation. Crosby acknowledged the cavelike feature suggests a complex system of knickpoint advancement in this setting. “apparently impotent little brook” (p. 471) in the headwaters, but credits weather- of channel extension “exhausts” these lumped under hillslope processes. ing between floods for wearing away factors. Because of the inertia of the system, these rocks and the floods themselves groundwater sapping is more continu- for removing sediment. STREAM PIRACY: ous and persistent over time as com- There are many complicating factors GROUNDWATER AND SURFACE pared to surface water. in determining the mechanisms and WATER AS COOPERATORS Groundwater energy is little affected rates for channel extension. Weissel and Water, the common element of surface by topographic divides as compared to Seidl (1997) found that while bedrock water and groundwater, is obviously the surface-water energies. Surface-water lithology and upstream drainage area major initiator and accelerator in erosion energy usually decreases near divides had minimal impact on knickpoint and the key element in stream piracy. because of shrinking catchment areas retreat, bedrock jointing profoundly The hardest rock can be broken down and sometimes decreased surface gradi- affected the hillslope processes that con- by freeze-thaw cycles, and a cohesive ents. With breaching of the divide, control knickpoint migration rates. Whipple sediment can be fragmented by wet-dry siderable erosion would be required to et al. (2000) determined that the efficacy cycles. Fractures in an “impermeable” develop a catchment area for surface of fluvial erosion processes (plucking, rock leave them vulnerable to erosion runoff to feed the pirating stream on the abrasion, cavitation, and solution) was a and enlargement by water. Water sup- pirated stream side of the drainage strong function of substrate lithology ports living organisms that condition basin. This is unlikely because nearly all and that joint spacing, fractures, and rock and sediment for erosion, and it sediment comes from the extending bedding planes exert the most direct can dissolve interstitial cement and the channels. If there is insufficient flow in control. Montgomery and Dietrich (1989) rock itself. Where there is water move- the channel, accumulating sediment in identified thresholds of upslope catchment, there is an increasing probability the headcut can slow the erosion pro- ment area needed to erode a channel of a chemical disequilibrium between cess, so availability of sediment transport head. As a channel head approaches a the rock and the interstitial water. Posi- may control the rate of extension. This divide, the catchment area will decrease, tive pore-water pressure undermines fact is an argument for the occurrence of reducing runoff. Gomez and Mullen slopes, triggers landslides, and con- stream piracy being most likely during (1992) recorded more than 90% of net- tributes to debris flows. As water erodes, wetter climatic periods. Finally, exten- work growth in the first 10% of their it usually creates a preferred flow path, sion of a channel is often very rapid at experiment’s duration. An exponential further accelerating the erosion process. first, implying that a threshold has been curve for gully extension was suggested Groundwater-sapping processes are crossed. This also suggests that the final by Rutherford et al. (1997). A broader equally effective in the presence of sur- act of stream piracy should occur as the interpretation is that intrinsic and/or face water, but they are often not recog- consequence of an event rather than as extrinsic factors may provide a threshold nized because either the evidence is the continuation of an average. for initiation, and the physical effect destroyed by surface water or they are The only connection between the

8 SEPTEMBER 2001, GSA TODAY pirated and incipient pirating stream is ing sediment and rock. The presence of has the potential for pre-piracy “commu- through the groundwater-flow system perennial surface water is strong evi- nication” between the pirating and (Fig. 3). The incipient pirating stream dence for groundwater intersecting the pirated stream (Fig. 3). The pirated can gain flow (sapping energy) from the surface and for effective groundwater stream can provide flow and potential pirated stream. The pirating stream can sapping. Groundwater sapping is effec- energy to the groundwater system. This cause the accumulation of sediment in tive in the absence of surface water. can further enhance groundwater sap- the incipient pirated stream. This con- Unfortunately, the evidence for ground- ping at the headcut of the extending cept should apply in the building and water sapping is usually destroyed by pirating stream. As the pirating stream breaching of river levees, delta formation, surface water. advances, the changing groundwater- alluvial fans, and hillslope processes. Headward extension of channels flow paths (Fig. 3) can cause a loss of Most stream piracy is likely the result results in a distortion of the groundwa- flow in the pirated stream, resulting in of a succession of channel extensions in ter-flow system. The head of the channel sediment accumulation in the bed as response to climatic events. One must represents a low potential energy point stream competency decreases. This can recognize that the current climatic condi- for groundwater flow, much like a pump- in turn lead to an increasing energy gra- tions may not be the same as when ing well. With concentration of flow, dient in the groundwater-flow system. stream piracy occurred. Just as short- groundwater-sapping processes are con- A heterogeneous and anisotropic geo- term precipitation events accelerate centrated at the headcut, resulting in fur- logic environment would modify the groundwater-sapping erosion and sur- ther channel extension and/or incision actual groundwater-flow paths, but the face-water erosion, longer climatic that in turn leads to increasing incision overall results would be similar to the events can add or remove energy from of the groundwater-flow system. There homogeneous and isotropic model the pirating equation, including base- is the potential for the development of shown (Fig. 3). Karst development would level changes. Higgins (1984) proposed thresholds that, on being exceeded, can occur along fractures and higher perme- that the pectinate (comb-like) drainage lead to rapid channel extension. An ability zones, reflecting the imprint of networks of the High Plains were analogous event would be the failure of the groundwater-flow system from re- formed chiefly by groundwater sapping the Teton Dam. charge to discharge areas. Fractures zones when the water tables were higher dur- Groundwater sapping should be sus- would enlarge and grow in a similar ing wetter climates of the past. Alley pected at locations where the surface manner. (2000) highlighted the relative stability of morphology suggests incision into the Because groundwater sapping is a the climate over the past 10000 yr as groundwater table and concentration of basic geologic process, and energy gra- compared to the much larger instabilities groundwater flow. A greater potential for dients can exist at all scales, the stream- of the past 100000 yr, so our historical groundwater sapping is found at the piracy model presented in this paper perceptions may be inappropriate in outside of meander bends, at the point should be applicable at all time and size interpreting stream piracy. The rates of where streams become live, where river- scales. Stream piracy is a possibility on surface processes and denudation banks feel spongy, where talus slopes the smallest streams and the largest (Young and Saunders, 1986) are such form during freeze-thaw cycles, and in intermountain drainage basins. Where that many stream-piracy events likely deeply entrenched channels. Groundwa- diversion has occurred, groundwater occur over time scales greater than the ter sapping may be found in areas where sapping should be suspected. current period of climate stability, espe- banks and slopes are wet or undercut or This model suggests that higher cially in lithified and cohesive material. have zones of vegetation growth, surface energy gradients would be expected However, of all natural variables control- evaporitic deposits, desiccation cracks, during wetter climatic periods, and as a ling surface water and groundwater flow water flow from fractures, and evidence consequence, channel extension should in general, only climate can change sig- of soil flow. If the setting appears too dry, be more rapid during these periods. The nificantly over time periods shorter than try visualizing a long-duration thunder- increased rate of channel extension under geologic time. An exception to the pre- storm and a much higher groundwater wetter climatic conditions is intuitive in vious statement may occur with disrup- table. part, but by extension, it also means that tions of drainages during earthquakes, If there is a difference in elevation of the actual stream diversion is most likely volcanic activity, and subglacial drainage streams in adjacent drainage basins, the during wetter climatic periods. events. On an even shorter time scale, topographic divide does not correspond Lane (1899) promoted the role of several years of unusually high precipita- to the groundwater divide (Fig. 3). As a groundwater sapping in stream piracy. tion can significantly increase the erosive result, the groundwater-flow system main- Unfortunately, the visibility of surface power of groundwater and surface water. tains its energy as the headcut approaches water and the lack of understanding of and crosses the topographic divide, be- groundwater-flow and groundwater-sap- SUMMARY cause its energy comes from the ground- ping processes have led to a long advo- Groundwater exists in nearly all geo- water divide. In contrast, the energy of cacy of surface water as the pirating vil- logic environments in a dynamic system surface water at the headcut decreases lain. Waterfalls (e.g., Fig. 4; cover photo) with persistent flow from recharge areas as the divide is approached because of on the Island of Hawaii demonstrate that to discharge areas where groundwater- reduced catchment area and possibly factors other than surface water alone sapping processes are focused. Ground- reduced surface gradients. must be at work in knickpoint migra- water sapping is highly effective in erod- Only the groundwater-flow system tion. There is considerable evidence for

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Whipple, K., Hancock, G., and Anderson, R., 2000, River Bishop, P., 1995, Drainage rearrangement by river capture, Kochel, R., Simmons, D., and Piper, J., 1988, Groundwater incision into bedrock: Mechanics and relative efficacy of beheading, and diversion: Progress in Physical Geography, sapping experiments in weakly consolidated layered sedi- plucking, abrasion and cavitation: Geological Society of v. 19, p. 449–473. ments: A qualitative summary, in Howard, A., et al., eds., America Bulletin, v. 112, p. 490–503. Sapping features of the Colorado Plateau: A comparative Young, A., and Saunders, I., 1986, Rates of surface pro- Butt, T., and Russell, P., 2000, Hydrodynamics and cross- planetary geology field guide: Washington, D.C., National cesses and denudation, in Abrahams, A.D., ed., Hillslope shore sediment transport in the swash zone of natural Aeronautics and Space Administration, p. 84–93. beaches: A review: Journal of Coastal Research, v. 16, processes: The Binghamton Symposia in Geomorphology, p. 255–268. Laity, J., and Malin, A., 1985, Sapping processes and the International Series 16: Boston, Massachusetts, Allen & development of theater-headed valley networks on the Unwin, p. 3–27. Collison, A., 1996, Unsaturated strength and preferential Colorado Plateau: Geological Society of America Bulletin, flow as controls on gully head development, in Anderson, v. 96, p. 203–217. Manuscript received March 26, 2001; M., and Brooks, S., eds., Advances in Hillslope Processes: New York, John Wiley & Sons, p. 753–769. Lane, A., 1899, A note on a method of stream capture: Geo- accepted July 6, 2001. logical Society of America Bulletin, v. 10, p. 12–14. Crosby, I., 1937, Methods of stream piracy: Journal of Geology, v. 45, p. 465–486. Lawler, D., 1993, Needle ice processes and sediment mobi- lization on riverbanks: The River Ilston, West Glamorgan, DeVries, J., 1976, The groundwater outcrop-erosion model: United Kingdom: Journal of Hydrology, v. 150, no. 1, Evolution of the stream network in the Netherlands: Journal p. 81–114. of Hydrology, v. 29, no. 1-2, p. 43–50. Mather, A., Harvey, A., and Stokes, M., 2000, Quantifying DeVries, J., 1994, Dynamics of the interface between stream long-term changes of alluvial fan systems: Geological Society GSA is looking for a co-editor for GSA Today, to and groundwater systems in lowland areas, with reference of America Bulletin, v. 112, p. 1825–1833. to stream net evolution: Journal of Hydrology, v. 155, no. 1-2, serve a three-year term beginning in January p. 39–56. Montgomery, D., and Dietrich, W., 1988, Where do channels begin?: Nature, v. 336, p. 232–234. 2002, as one of a two-editor team. Desirable DeVries, J., 1995, Seasonal expansion and contraction of stream networks in shallow groundwater systems: Journal Montgomery, D., and Dietrich, W., 1989, Source areas, characteristics for the successful candidate of Hydrology, v. 170, no. 1-4, p. 15–26. drainage density, and channel initiation: Water Resources Research, v. 25, p. 1907–1918. include: broad interest and experience in geology; Dunne, T., 1980, Formation and controls of channel networks: Progress in Physical Geography, v. 4, p. 211–239. Nash, D., 1996, Groundwater sapping and valley develop- familiarity with many earth scientists and with ment in the Hackness Hills, North Yorkshire, England: Earth new trends in science; and expertise which Dunne, T., 1990, Relation of subsurface water to downslope Surface Processes and Landforms, v. 21, p. 781–795. movement and failure, in Higgins, C.G., and Coates, D.R., complements rather than duplicates that of eds., Groundwater geomorphology: The role of subsurface Netto, A., Fernandes, N., and de-Deus, C., 1988, Gullying water in earth-surface processes and landforms: Boulder, in the southeastern Brazilian Plateau, Bananal, SP: Interna- co-editor Karl Karlstrom (structure and tectonics). Colorado, Geological Society of America Special Paper 252, tional Association of Hydrological Sciences Publication p. 51–76. 174, p. 35–42. GSA provides a small stipend and expenses for Dunne, T., 1998, Critical data requirements for prediction of Norris, R., and Back, W., 1990, Erosion of sea cliffs by mail and telephone. erosion and sedimentation in mountain drainage basins: groundwater, in Higgins, C.G., and Coates, D.R., eds., Journal of the American Water Resources Association, v. 34, Groundwater geomorphology: The role of subsurface water Submit a c.v. and a letter describing why you’d p. 795–808. in earth-surface processes and landforms: Boulder, Colorado, Geological Society of America Special Paper 252, like to be co-editor (or, if nominating another, Freeze, R.A., 1987, Modeling interrelationships between cli- p. 283–290. mate, hydrology, and hydrogeology and the development submit a letter of nomination and the person’s of slopes, in Anderson, M.G., and Richards, K.S., eds., Slope Pederson, D.T., 1995, Pattern of stream development in the written permission) to Jon Olsen, Director of stability: New York, John Wiley & Sons, p. 381–403. Nebraska Sand Hills and controlling processes [abs.]: EOS (Transactions, American Geophysical Union), v. 76, p. 267. Fyodorova, A., and Sasowsky, I., 1999, Silica dissolution Publications, P.O. Box 9140, Boulder, CO and the development of sandstone caves: Geological Society Pissart, A., Krook, L., and Harmand, D., 1997, The capture 80301-9140, or [email protected], by of America Abstracts with Programs, v. 31, no. 7, p. A51. of the Aisne and heavy minerals in the alluvium of the Meuse in the Ardennes: Comptes Rendus de l’Academie des October 31, 2001. Gabbard, D., Huang, C., Norton, L., and Steinhardt, G., Sciences, Serie II, Sciences de la Terre et des Planetes, 1998, Landscape position, surface hydraulic gradients, and v. 325, p. 411–417.

10 SEPTEMBER 2001, GSA TODAY