. t 

FINAL CONTRACTORS REPORT 

h 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

SOLAR SYSTEM EXPLORATION DIVISION 

PLANETARY GEOLOGY AND GEOPHYSICS PROGRAM 

N.A.S.A. GRANT - NAGW 707 

(FEBRUARY 1,1085 - JANUARY 31,1988) 

GROUNDWATER SAPPING VALLEYS: EXPERIMENTAL STUDIES, 

C EOLO(; ICAL CON'I'ROIS, AND IMI'LICA'I'IONS 'I'O '1'1 I15 

IN'I'EH PIW'I' AT IO N 0 17 VALLEY N El' W 0 R ICs ON klAI<S 

PRINCIPAL INVESTIGATOIZ: 

R. CRAlG KOCMEL 

Surficinl Processes Reserrrdi Croup 

Department of Geology 

Southern Illinois University at Carbondale 

Cu.bondriie, Illinois 62001 

May 15,1988 

(NASA-CB- 7 827 18) G BO UNDRATEB S APPI MG ~08-22a4-1 

VALLEYS: EXPERIMENTAL STUDIES, GEOLOGICAL 

CONTROLS &ND XVPLICATIONS TO THE 

INTERPRETATION OF VALLEY NETWORKS ON MARS 

Final Contractor's Report, 1 Peb. (University G3/91 

3ncl as 

136187 
FINAL CONTRACTORS REPORT 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

SOLAR SYSTEM EXPLORATION DIVISION 

PLANETARY GEOLOGY AND GEOPHYSICS PROGRAM 

N.A.S.A. GRANT - NAGW 707 

(FEBRUARY 1,1985 -JANUARY 31,1988) 

GROUNDWATER SAPPING VALLEYS: EXPERIMENTAL STUDIES, 

GEOLOGICAL CONTROLS, AND IMPLICATIONS TO THE 

INTERPRETATION OF VALLEY NETWORKS ON MARS 

PRINCIPAL INVESTIGATOR: 

R. CRAIG KOCHEL 

Surficial Processes Research Group 

ucpsu ullt;Ill Ul ueu1ugy 

Southern Illinois University at Carbondale 

Carbondale, Illinois 6290 1 

n------. -cn--i--- 

May 15,1988 

1 
TABLE OF CONTENTS 

TOPIC PAGE NUMBER 

ABSTRACT ................................................................. 3 

ACKNOWLEDGEMENTS .................................................. 3 

INTRODUCTION ............................................................. 4 

Channels on Mars .................................................... 4 

Groundwater Sapping ............................................... 5 

Outline of Report ..................................................... 6 

EXPERIMENTAL SAPPING VALLEYS ................................. 

Preliminary Experimental Studies .................................. Experimental Sapping at SIU ....................................... Experiments in Homogeneous Sediment ................. Experiments in Layered Stratigraphy ..................... Expeiments Using Joints ................................... Experiments With High-Level Aquifers .................. Alcoving Experiments ...................................... 

Objectives ............................................................. 

Summary of Flume Experiments ................................... 

TERRESTRIAL ANALOG STUDIES OF SAPPING VALLEYS ..... Hawaiian Valleys ..................................................... rid Reconnaissaiiee ....................................... Remote Studies .............................................. Valley Evolution ............................................. Colorado Plateau Valleys ............................................ 

v* -1 

Distinguishing Runoff vs . Sapping Valleys .............. Geological Controls on Sapping Dominated Valleys .... 

8 

8 

8 

13 

20 

31 

37 

41 

48 

48 

49 

49 

54 

56 

58 

60 

63 

m 

SUMMARY .................................................................... 72 

REFERENCES ................................................................ 74 

2 
ABSTRACT 

An integrated approach using experimental laboratory models, field studies of 

terrestrial analogs, and remote studies of terrestrial field sites were applied to the goals of 

understanding the nature and morphology of valley networks formed by groundwater 

sapping. In spite of problems with scaling, the experimental studies provided valuable 

insights into concepts relating to the initiation, development, and evolution of valleys by 

groundwater sapping. These investigations were also aimed at developing geomorphic 

criteria for distinguishing valleys formed dominantly by surface runoff from those formed 

dominantly by groundwater sapping processes. Channels that were field classified as 

sapping vs. runoff were successfully distinguished using statistical analysis of their 

respective morphologies, therefore, it may be possible to use similar techniques to interpret 

channel genesis on Mars. The terrestrial and flume studies provide the ground-truth dataset 

which can be used (and will be during the present year) to help interpret the genesis of 

valley networks on Mars. 

Our studies provided a conceptual model of how sapping dominated valleys evolve, 

and elucidated our understanding of the geologic controls affecting valley morphology. 

Better understanding of these controls may enable us to make reliable interpretations about 

the underlying geologic structure and composition of selected areas on Mars. Many of the 

results from this project have been published or are currently in press, therefore, they will 

only be summarized here. 

ACKNOWLEDGEMENTS 

This research was funded by N.A.S.A. Grant NAGW-707 over three years 

between Fehrw~y 1, 1OSC tn Jrrnuary 31,1088, with e~pefimentd resea-ch rnntinuing into 

late March, 1988. I wish to thank the three technical monitors at NASA headquarters for 

their help in facilitating the research - initially Dr. Joe Boyce, followed by Dr. David Scott, 

and currently Dr. James Underwood, Jr. Research assistants during the first half of this 

project deserve special thanks for working in difficult conditions - Mr. Jay Piper and Mr. 

Dave Simmons. Mike Phillips helped in the field and lab research during the last year. 

Many other SIU graduate students freely contributed their time to the labor intensive 

task of obtaining changing, and evacuating sediment from the experimental fluvial 

laboratory. The Department of Geology frequently covered expenses associated with the 

logistics of running the experiments and obtaining sediment for the experiments. 
MARTIAN VALLEY TYP ES 

SIZE MORPHOLOGY DISTRIBUTION 

10's km long anaslomosis,-grwves, mostly equatorial 

iG's km wide sireamiined hills, regions 

OUIFLOW massive flood features, 

CHANNELS distinct chaos sources, 

lack tributaries 

I)HY 10's km long dendritic pattern wl ancient hilly & 

VALLEYS <few km numerous tributaries, cratered terrain 

wide fresh & weathered 

INTRODUCTION 

CHANNELS ON MARS 

Studies of the Viking images of Mars have resulted in the identification of at least 

four major classes of Martian channels (Figure 1) (See reviews by Baker, 1982 and Mars 

Channel Working Group, 1983). Enormous channels hundreds of kilometers long and 

tens of kilometers wide were extensively mapped and interpreted as having formed by 

cataclysmic flooding (Baker and Milton, 1974; Baker and Kochel, 1978a, 1978b, 1979; 

Masursky and others; 1979; Grant, 1987). Termed outflow channels, these large Martian 

channels contain an assemblage of depositional and erosional features analogous to the 

largest known terrestrial cataclysmic floods which coursed through the Channeled 

Scablands of eastern Washington numerous times during the Pleistocene (Baker, 1973; 

Baker and Nummedal, 1978; Baker, 1978). 

Recently, the focus of attention in Martian channel studies has turned to an 

intriguing array of smaller-scale channel features generally integrated into drainage network 

patterns (See review in Baker, 1982). Mars valley networks can be divided into three 

general styles (Figure 1): 1) dry valley networks which have distinctly dendritic tributary 

patterns formed generally on ancient hilly and cratered terrain; 2) longitudinal valley 

networks which have short, stubby tributaries with amphitheater heads; and 3) slope valley 

networks which have short, amphitheater-headed tributaries formed along major 

escarpments such as Valles Marineris and on the slopes of major volcanic cones. 

LONGI- 100's km long flat-floored main mostly equatorial 

TUDINAL few km wide channel, short & regions VALLEYS stubby tributaries, 

amphitheater heads 

SLOPE 10's km long flat-floored & equatorial along 

VALLEYS few km wide V-shaped cross- Valles Marineris 

sections, amphitheater chasma 

heads, stubby tribs. 

w/ high junction <'I 

Figure 1. Classification of Martian channels visible in Viking images. Valley networks 

and some margins of outflow channels are the focus of studies related to sapping 

Theories explaining the origin of Martian valley networks exhibit considerably less 

agreement than models suggested for the outflow channels on Mars. The occurrence of dry 

valley networks on very ancient terrains contemporaneous with the late stages of planetary 

accretion (Baker and Partridge, 1986) place their origin during a period of Martian history 

that many speculate was characterized by a climate capable of sustaining significant 

precipitation. The superficial resemblence of dry valley networks to terrestrial dendritic 

4 
I 

valley networks formed by precipitation and runoff led to suggestions that the Martian 

valley networks were formed in the same manner (Milton, 1973, Sagan and others, 1973; 

Sharp and Malin, 1975). Although there is support for a more volatile rich early Martian 

atmosphere (Pollack, 1979; Toon and others, 1980) with considerable volumes of initial 

water (Squyres, 1984), there are enough conflicting ideas and models of early conditions to 

warrant caution to be exercised in using the presence of valley networks as independent 

evidence of a former warmer climate capable of the kind of intense precipitation that would 

be necessary to develop valleys of the dimensions present on Mars. 

During the past few years, investigators of Martian channel networks realized these 

valleys had peculiarities in their morphologic expression compared to terrestrial runoff 

systems. In particular, attention was called to the amphitheater-heads, apparent low 

drainage density, stubby tributaries, and structural control of Mars valley networks which 

led to suggestions that perhaps the process of groundwater sapping played a significant role 

in creating these networks (Baker, 1982; Baker and Kochel, 1979; Higgins, 1982; 1984; 

1988; Kochel and others, 1983; Kochel, Howard, and McLane, 1985; Laity and Malin, 

1985; Howard and McLane, 1981; Howard and others, 1988). Additional studies of slope 

degradation on Mars have also invoked groundwater sapping as a major process which 

may have contributed to backwasting and downwasting of Martian landforms (Sharp, 

1973; Baker and Kochel, 1979; Kochel and Baker, 1981; Kochel and Burgess, 1983; 

Kochel and Capar, 1983; Kochel and others, 1983; Kochel, 1984; Kochel and Peake, 

1984). Groundwater sapping is presumed to be a reasonable process which may have 

operated on Mars due to its ability to operate in permeable materials by recharge with low 

intensity precipitation. Alternatively, sapping could even operate in the absence of 

precipitation by the thermal decay of ground-ice due to a host of triggering mechanisms 

unrelated to precipitation. 

GROUNDWATER SAPPING 

Most terrestrial channels are probably formed and maintained by a combination of 

runoff and groundwater sapping processes, therefore, are best referred to as composite 

channels (Schumm and Phillips, 1986). Prior to the late 1970s most models of channel 

formation focused upon erosional processes associated with Hortonian overland flow. 

Recently, geomorphologists and hydrologists have begun to recognize the importance of 

subsurface flow processes on the initiation and maintenance of surface channels. 

Groundwater sapping is a rather broad term which refers to the weathering and erosion of 

sediments and rocks by emerging groundwater by some combination of intergranualr flow 

and channelized flow (referred to as piping). Groundwater sapping discharge typically 

emerges as distinct springs, referred to as spring sapping or along laterally extensive planar 

zones where it is referred to as seepage erosion (Higgins, 1984,1988). 

Dunne (1980) provided an excellent review of channel initiation processes by 

sapping and runoff and illustrated clearly how effective sapping can be in creating and 

extending terrestrial channel networks. Dunne (1980) depicted the random emergence of a 

spring along an escarpment and showed how it is rapidly followed by groundwater flow 

convergence into the spring head as it retreats headwardly (Figure 2). This flowline 

convergence operates as a positive feedback process which favors continued expansion of 

the selected springhead and valley at the expense of neighboring regions of the escarpment 

which have large portions of their groundwater recharge diverted into the favored channel 

by subsurface piracy. Through competition, selected channels are enlarged while 

neighboring channels become less active or relict. 

5 
(a) %/ / (b) I -IJ\ 

Figure 2. Scenario model of the development of channel networks by groundwater 

sapping. Erosion by rivers or landslides results in an irregularity along the escarpment. 

The cut will favor headward channel extension because of favored flow convergence of &*"U..U. mminrlr\iatPr ULVA. PP .." fawnring LUIVAA.. is pregrerpysrpysi\fp_. (fr~m Dunfip_, 1988). 

Since the suggestion that sapping processes may be important in Martian channel 

formation, the importance of sapping processes as a geomorphic agent of erosion has 

become increasingly evident in many terrestrial physiographic settings, on a variety of 

scales, and in a variety of unconsolidated and consolidated geologic materials (Higgins, 

1982; 1984; Baker; 1982; Kochel and others, 1985; Laity and Malin, 1985; Pillans, 1985; 

Kochel and Piper; 1986; Schumm and Phillips, 1986). Table 1 provides a review of 

significant recent studies involving groundwater sapping in terrestrial and planetary 

settings. 

OUTLINE OF THE REPORT 

The results of our three-year investigation of groundwater sapping studies, 

summarized in the following report, represent the integration of a four-phased research 

approach. Much of the research reported here has been done in collaboration as a 

coordinated effort with Alan Howard of the University of Virginia. First, the report will 

summarize the results of a series of experiments in sapping tanks designed to provide a 

conceptual framework for understanding the development and evolution of valley networks 

formed by groundwater sapping. These experiments also contribute to models for 

comparing sapping and runoff valleys. Second, we will summarize a series of studies of 

terrestrial analog sites where sapping processes dominate channel evolution using a 

combination of field reconnaisance observations and aerial photographic studies to develop 

a suite of geomorphic criteria useful in distinguishing between sapping and runoff valley 

networks. Third, we will summarize field and experimental studies aimed at determining 

6 
the important geological controls on sapping processes and how these controls may be 

reflected in valley network morphometry and channel morphology. The compliment to this 

aspect of our research is an attempt to show how morphologic expression of sappingdominated valleys can be used to interpret the nature of geological materials on which they 

are developed. 

The last phase of the research described in this report leads directly into the research 

currently underway to apply the experimental and terrestrial analog results to the 

interpretation of the genesis of Martian valley networks. The intent of the current research 

effort is to develop a global map of Mars channels classified by genesis. In addition, we 

plan to use to results of the studies summarized in this report to make interpretations about 

the underlying geology for selected valley network regions of Mars. 

Table 1. Recent Major Published Groundwater Sapping Studies 

Author Date Focus of the Contribution 

R.P. Sharp 1973 

V.R. Baker, R.C. Kochel 1979 

T. Dunne 1980 

V.R. Baker 1982 

C. Higgins 1982 

Mars Channel Group 1983 

R.C. Kochel, A.D. Howard 1985 

J. Laity, M. Malin 1985 

Howard, Kochel, Holt 1985 

1988 

R.C. Kochel, J. Piper 1986 

V.R. Baker, J. Partridge 1986 

S.A. Schumm, L. Phillips 1986 

R.C. Kochel and others 1988 

R.C. Kochel, G. Riley 1988 

A.D. Howard 1988 

C. Higgins 1988 

~ 

Suggested sapping processes to explain some Mars terrains 

Sapping suggested to explain channel walls on Mars 

Formation of terrestrial channel networks & theory 

Review of Mars channels and valley networks - book 

Analogy of beach sapping tidal channels (micro) to Mars 

Review of Mars channels and networks 

Experimental sapping valleys and Mars analogies 

Sapping processes and Colorado Plateau 

Groundwater sapping field conference - NASA - 11/85 

publication of guidebook 

Distinguish Hawaiian sapping & runoff valleys - Mars 

Dry Mars valleys, degraded and pristine and sapping 

Discuss composite origin of terrestrial channels 

Summary of expeimental sapping studies in sediments 

Sedimentary controls sapping Colorado Plateau 

Theoretical review of sapping processes & experiments 

Review book on groundwater sapping - many authors 

7 
Table 2 provides a summary of the publications and manuscripts in press which 

have resulted to date from this contracted research. Numerous additional manuscripts are 

currently in preparation and planning related to this research both jointly and separately 

between our efforts at SIU and UVa. 

EXPERIMENTAL SAPPING VALLEYS 

OB JECTNES 

The primary purpose for the experimental phase of this research was to design a 

laboratory system which would be conducive to the development of surface valley 

networks by groundwater sapping processes. Experimental study of small-scale sapping 

networks allows direct observation of valley evolution and provides data on basin 

morphometry and channel morphology of these systems which can be compared to 

terrestrial runoff valley networks and used in interpretation of genesis of Martian valleys 

networks. 

The major objective of the sapping experiments was to provide a database for 

testing conceptual and theoretical models of the development of valley networks by 

groundwater sapping. In particular the secondary objectives included: 1) characterization 

of the morphology of sapping valleys and morphometry of sapping valley networks formed 

in a variety of unconsolidated and weakly consolidated sedimentary materials; 2) 

characterization of the differences in sapping networks developedin uniform, multi-layered 

strata, and in experiments simulating numerous structural perturbations such as joints, 

dikes, and folding; 3) development and description of the evolution of sapping valley 

networks; 4) morphometric comparison of experimental sapping and runoff valley 

netwnrks; ar?d 5) ccmp~4ism of experiments! ssi;i;iiig fietwoiks io selected vaiiey 

networks on Mars. A pimary factor in the selection of variations to the experimental design 

was the attempt to simulate certain aspects of the geologic setting of the areas where we 

were conducting terrestrial analog studies of sapping-dominated valleys. Finally, the 

experimental data will provide valuable calibration data for the theoretical modeling of 

sapping valley systems being conducted simultaneously by Alan Howard at University of 

Virginia. 

PRELIMINARY EXPERIMENTAL STUDIES 

The combination of simultaneous experiments using a two-dimensional sapping 

tank by Howard (Univ. Virginia) and a small 3-dimensional experimental tank less and 0.8 

cubic meter in size by Kochel (then at SUNY Fredonia) showed that channel networks 

could be developed by groundwater sapping processes in the absence of surficial runoff in 

unconsolidated sediments. These early experiments provided the impetus for the 

elaboration and design of a new, larger sapping tank having dimensions of about 1 cubic 

meter at University of Virginia by Howard and Kochel in 1983. Finally, the contracted 

research reported on here was conducted in a larger flume modified for sapping 

experiments to extend over an area 2.5 m wide by 5 m or more long, and slightly less than 

1 meter deep at Southern Illinois University. 
2. D-HY OF PUBSUPPORTED BY THIS CON TRACT bv R. Craie Koclid 

ls.5 

Kochel, R. C.. 1985, Martian fretted tcnain morphometry interpreted using principal components analysis: NASA Technical Memorandum No. 87563. p. 352-354. 

Kochel, R. C., and Iloward, A. D.. 1985. Groundwater sapping and ancient valley nctworks on Mars: Lunar and Planerruy Science Institute Technical Report No. 8543, p. 48-50. 

Kochel, R. C., Howard, A. D., and McLane, C., 1985, Channel nctworks developed by 

groundwater sapping in he-grained sediments: analogs to some Martian valleys: in, 

Woldenberg. M. (Ed.), Models in Geomorphology: Roston, Allen & Unwin, p. 31 3-341. 

Kochel, R. C., 1985. Role of groundwater sapping in development of large valley networks on Hawaii: in. Howard, A. D.. Kochel. R. C., and IIolt. 11.. (ds.) Field Guide and Programfor 

NASA Groundwater Sapping Conference -Preliminary Draji, November, Flagstaff, p. 14-16. 

Groundwater Sapping Conference -preliminary drgft. Fagstaff, 76p. 

IIowd, A. D., Kochel, R. C., and IIolt. H., 1985, Field Guide and Programfor NASA 

1ps6 

Kochel. R. C., and Piper, J. F., 1986, Morphology of large valleys on 1Iawaii: Implications for 

groundwater sapping and comparisons to Martian valleys: Abstracts. 17th Lunar and Planetary 

Science Conf., p. 424-425. 

groundwater sapping and comparisons with Martian valleys: Journal of Geophysical Research, 

Kochel, R. C., and Piper. J. E, 1986. Morphology of large valleys on Hawaii: Evidence for 

V. 91,~. E175-E192. 

Kochel, R. C., 1987, Valley morphology on Hawaii and Mars: Arguments for their origin by 

groundwater sapping processes: Geological Society of America. Abstracts with Progr~t?~~, v. 19, no. 6, p. 395-3%. 

Kochel, R. C.. and Phillips. M. A., 1987. Geological controls on the morphology of valleys 

influenced by groundwater sapping on the Colorado Plateau: Geological Society of America 

Abstracts with Programs, v. 19, no. 7. p. 731. 

Kochel, R C.. and Simmons, D. W., 1987, Groundwater sapping channels: Summary of effects 

of experiments with varied stratigraphy: NASA Technical Memorandum No. 89810, p. 

291-293. 

Kochel, R. C., and Phillips, M. A., 1988, Preliminary investigation of geological controls on valleys influenced by groundwater sapping, southern Colorado Plateau, Arizona and Utah: 

Abstracts, 19th Lunar and Planetary Science Coqerence, p. 61 5-61 6. 

In press for 1988: 

Howard, A. D., Kochel. R. C., and Holt, H.. 1988, Proceedings and Field Guide for the NASA 

Groundwater Sapping Conference, Washington, NASA, 208pp. 

Howard. A. D.. and Kochel, R. C.. 1988. Introduction to Cuesta Landforms and Sa ping 

Processes on the Colorado Plateau: in. Iloward. A.D.. Kochel. R. C.. and Ilolt, fr.. 

(editors)Proceedings and Field Guide for the NASA Groundwater Sapping Conference, 

Washington, NASA, p. 1-91. 

Kochel, R. C., Simmons, D. W., and Piper, J. E, 1988. Groundwater sapping experiments in 

wcakly consolidated layed sediments: A qualitative summary: in. Iloward. A.D.. Kochel, 

R.C.. and IIolt. ll., (editors) Proceedings andField Guide for the NASA Groundwater 

Sapping Conference, Washington, NASA, p. 161-183. 

Kochel. R. C., and Riley, G. W.. 1988. Sedimentologic and stratigraphic variations in sandstones 

of the Colorado Plateau and their implications for gmundwatcr sapping: in. Iloward. A.D., 

Kochel, R.C., and Holt. H.. (editors) Proceedings and Field Guide for the NASA Groundwoler 

Sapping Conference, Washington, NASA, p. 102-1 15. 

Kochel, R. C., and Baker, V. R.. 1988. Case Study: Groundwater sapping and the development 

of large Hawaiian valleys: in, Spring Sapping and Valley Network Development. Chapter 10, 

in, IIiggins. C.G.. and Coates, D.R., (editors) Groundwater Geomorphology. Geological 

Society of America Special Paper, in press. 
I 

r 

I 

I 

I 

I 

! 

I 

i 

I 

Design of Earlv Exueriments 

Experiments in the Fredonia sapping box were confined to homogeneous sediments 

composed of a mixture of 90% 2.25 phi fine sand and 10% clay-sized coal fly ash which 

helped provide cosheion for channel walls (Kochel, Howard, and McLane, 1985). The 

purpose of these experiments was to determine if surface channels could be created by 

groundwater sapping processes in the total absence of runoff. Constant head was 

maintained throughout a series of 23 runs of less than 3 hours duration. Sapping channels 

evolved through a processes of rapid headward extension following toe scarp slumping at 

the onset of saturation of the sediments from the rear reservoir. Tributary extension was 

extensive only along headward reaches of the valley networks where supplies of 

groundwater were plentiful. Downstream reaches were unable to develop new tributaries 

during latter stages of the runs because available groundwater was being intercepted or 

pirated by upslope, rapidly-extending valley networks (Figure 3). Insignificant changes 

occurred in drainage density and channel length following a period of rapid adjustment, in 

contrast to the abstraction of tributaries observed by Schumm and others (1987) in 

experiments with valleys formed by runoff (Figure 4). Table 3 summarizes that final basin 

morpmetric data for the 69 networks developed in the small sapping box. 

RUN 15 

I 

Figure 3. Example of channel networks form Fredonia sapping box (See Kochel, 

Howard, and McLane 1985 for discussion). 

10 
0.1 

I I I I 1 1 I fa1 

- 

100- 

I 

M 

1 I 1 I I 

13E 0 .... 017E 

'x E---- K21D C----I20A' x, 

h 5 150, v T I I I 1 I I I I 1 Y 

E 

* 

Icn 

z 

W 

W 

(3 

U 

0 

- 

n 

a 

n 

$ 

W 

A 

(3 

z 

Q 

z 

0 

V 

Z 

3 

W 

W 

Q: 

CK 

W 

i= 

7 

k 

0 20 40 60 80 100 120 140 160180 

MINUTES SINCE SAPPING BEGAN 

MINUTES SINCE SAPPING BEGAN 

Figure 4. Examples of channel evolution in Fredonia sapping box. Note tendency to 

approach equilibrium after 100 minutes (from Kochel, Howard, and McLane, 1985). 

Table 3. Morphometry of Sapping Valleys Formed in Fredonia SaDDinP Box 

ffrom Kochel. Howard. and McLane. 1985) 

Parameter n Mean Standard deviation 

basin area (cd) 

basin shape 

lemniscate k 

IengtNwidth 

junction angles 

81 

%+e* 

dimensionless 

drainage density 

bifurcation ratio 

length main channel/ 

length first order 

tributaries 

Shreve magnitude 

Strahler order 

channel length ratios 

1 :2 

1 :3 

2:3 

basin aredchannel 

area 

interchannel 

spacing 

69 101.00 120.00 

69 

69 

22 1 

2.83 

2.43 

1.20 

0.11 

35.00 

44.00 

17.00 

18.00 

69 

56 

9.10 

2.93 

6.61 

1.01 

23 

69 

69 

1.50 

4.45 

1.94 

1.10 

3.28 

0.89 

55 

27 

21 

1.09 

1.37 

1.20 

0.45 

0.53 

0.52 

69 

57 

6.70 

9.20 

2.72 

2.80 
Exuerimental Morphometry and ComDarisons to Mars 

Channels formed along the escarpment in the early experiments occurred with rather 

uniform lateral spacing which averaged about 9 cm. This spacing was presumed to reflect 

the scale of local subsurface catchment area for each of the growing sapping valleys 

(Kochel, Howard, and McLane, 1985). Strahler orders for the sapping valleys never 

exceeded 3.0. Similar, low order valley networks were studied along the Valles Marineris 

escarpment on Mars (Figure 5) and compared to the valleys formed by these early sapping 

experiments (Kochel, Howard, and McLane, 1985). Table 4 summarizes the morphometric 

data collected from 70 slope valley networks along Valles Marineris for comparison with 

Table 3. 

The morphometric characteristics of the sapping valleys were similar to the slope 

networks on Mars. In particular, these similarities included: 1) elongate basins with 

tendency for rapid widening toward basin heads; 2) an abundance of short, stubby 

tributaries of Strahler order 1.0; 3) high ratios of channel length to tributary length; 4) low 

dimensionless drainage density compared to terrestrial runoff valley networks; 5) regular 

interbasin spacing along the escarpments; 6) a lack of regularity in junction angles 

throughout the network: 7) considerably low stream order for the size of the basins; 8) 

channels with steep walls and amphitheater heads; 9) valleys with broad U-shaped crosssections; 10) abundant valley segments with right-angle bends; and 11) large channel 

width to total basin area compared to terrestrial runoff valleys. For more elaboration on the 

results of the early research and theory, see Kochel, Howard, and McLane (1985). 

Table 4. Momhometry of Sapping Vallevs Along Valles M'uineris. Mars 

jfrom Koc'nei. Howard. and Mciane. i985) 

Paramctcr It Mean Standard Dcviation 

basin area (km') 70 603.00 627.00 

basin shape 

lemniscate k 70 2.33 0.75 

lengthlwidth 70 2.06 0.64 

01 42.00 20.00 

el+% 49.00 21 .oo 

drainage density' 70 12.61 9.53 

junction angles 275 

dimensionless 

length main channel/ 

length first order 

channels 27 3.89 0.85 

bifurcation ratio 55 3.53 1.46 

Shreve magnitude (M) 70 5.01 3.85 

Strahler order 70 1.74 0.85 

channel length ratios 

1:2 54 1.39 0.42 

1:3 24 1.69 0.38 

2:3 24 1.23 0.29 

area 70 3.40 1.85 

spacing 54 20.50 5.70 

basin aredchannel 

interchannel 
ORIG€NAC PAGE Is 

DE POOR QUALITY 

Figure 5. Low order slope valleys along Valles Marineris, Mars. 

Note that Strahler orders are less than 3.0 

EXPERIMENTAL SAPPING STUDIES AT SIU (Feb. 1985 - Mar. 1988) 

Over a period of three years between Spring 1985 and Spring 1988 a series of 56 

experiments were ciesigned to investigate the morpho!ogy and evddcn of valleys formed 

by groundwater sapping in unconsolidated and slightly cemented sediments. The major 

tasks investigated in the research included the following: 

1) geomorphologic characterization of sapping valleys formed in uniform, 

homogeneous sediments and their evolution 

2) comparison of sapping valley morphology with experimental and terrestrial 

valleys formed by runoff processes - part of the dataset used to 

develop a suite of geomorphic criteria useful in distinguishing valley 

types 

3) investigation of the effects of varying geology on the development and evolution 

of sapping valleys - in particular, variations in stratigraphy, 

cohesion, and the effects of various structural features 

4) simulation of selected field situations where terrestrial analog studies of sapping 

phenomena were being conducted 

5) the development of an experimental dataset to be used in theoretical modelling 

studies of sapping valley evolution being developed by Howard at 

Virginia 

Although experimental studies inevitably suffer from problems of scale and other 

difficulties that will be discussed later, they served as invaluable devices for helping to 

build conceptual models for the development and evolution of valleys by sapping 

processes. Natural sapping processes occur with extreme slowness and are typically not 

conducive to the collection of process-oriented observations over the lifespan of most 

research projects. Schumm and others (1987) discussed the advantages and disadvantages 

of experimental geomorphic research at length and concluded that in spite of the scaling 

13 
problems and difficulties in modelling field boundary conditions in the lab, there are major 

advantages to these kinds of experiments. Foremost of these advantages are the 

identification, direct observation, and measurement of processes under controlled 

conditions that can not be investigated in the field. In addition, Schumm and others (1987) 

underscore the utility of experiments for permitting studies of the evolution of physical 

systems and allowing the study of the reaction of the systems to controlled perturbations in 

the boundary conditions or process variables. They further note that careful observation of 

these systems may reveal processes or concepts previously unrecognized from field and 

theoretical study. Our experiences with sapping experiments underscore the conclusions 

made by Schumm and others (1987) and have provided us with a much better conceptual 

framework to apply to field and theoretical situations involving sapping processes. 

, 

SIU SaDDing Flume 

The 12m x 2.5m SIU recirculating flume was modified for groundwater sapping 

experiments by installing a perforated, screened headwall near the rear of the flume which 

permitted the development of a constant head reservoir (volume = 10 cubic meters) for 

purposes of recharging a groundwater table established in a sediment wedge downflume 

from the headboard (Figure 6). Although the sediment wedge could have extended for 

some 10m downflume, experiments were typically limited to sediment wedges between 3m 

and 5m in length. The large size of this flume provided several advantages over earlier 

experimental designs. Wall effects were minimized and there were no downflume barriers 

to interfere with fluvial transport of material away from the seepage or sapping face. This 

system is capable of modeling processes acting in a variety of natural settings by varying 

sediment characteristics such as permeability, porosity, cohesion, stratigraphy, and 

structure. Control on the reservoir head was maintained by an overflow pipe which was 

variabie in elevation. Runs requiring rdhfd were accomp~isfied with the use of higation 

spriklers installed on the lab roof. 

Figure 6. Sketch of SlU flume modified for sapping. 

Downstream sapping face can extend unimpeded for 8 meters farther if necessary. 

14 
Data collected for each of the successful sapping run is summarized in Table 5. The 

frequency of data collection depended upon the length of the run and the rate at which 

changes were occurring. Most successful runs contain data on the rates of channel 

evolution measured by changes in channel width, channel head extension, and channel 

gradient. Morphologic data was collected using a 3-dimensions system based on a cart 

mounted on steel rails to the top of the flume capable of 1 millimeter accuracy. Overhead 

and oblique photographs were also taken to record each experiment and for use in 

digitization of planimetric changes in the channels. Figure 7 provides a general summary in 

chronologic order of the 56 runs made between January 1985 and March 1988. Later 

discussions of the results will treat the runs in groups categorized by experimental design 

objectives. 

Our experimental studies were partly transport limited because of scaling problems 

inherent in the relative size of the sand compared to the dimensions of the channels 

produced and their associated discharges. A series of runs showed that due to scaling 

problems inherent in the ability of small discharges to remove sand grains by seepage 

erosion and transport them downstream, slopes on the sediment wedge had to exceed a 

threshold which was material dependent. Most of the experiments involved fine sand on 

which slopes had to exceed 6O for sapping channels to form. Variations in sediment 

wedge slope were installed during each run setup and could also be adjusted through a 

range of 1.50 using the hydraulic system enabling the entire flume to be tilted. We also 

realize that there are problems involved with chemical reactions between the groundwater 

and cement used in some of the runs. We were unable to effectively simulate the effects of 

chemical weathering processes along the seepage faces which probably play a large role in 

the erosion of sapping valleys in many environments. 

All runs with the exception of Runs 39-44,46,48, and 50-54 were designed for 

purposes of constructing channel and valley networks. The excepted runs were designed 

to investigate alcoving processes and will be discussed under a separate heading. 

15 
T 

r 

16 
T 

i 

I 

: 

..,. "", . tw1ar. 

& 11n.-gn1n.( M 

r-".-adn.4 u,l4 

u I. IO 

smimn 

D 11". mU.4 w I.... 

B- cmn.-xnl.ln.. d T- 0- ..l..r un lI.Lnt.1 Y w wm. - vw1w In I.l"t.4 11 

m I.. 11 

smllrmrl 

U?. am. 0.fI". .w r* I .... 

m. am. 

- mlw 

L ' . 

Figure 7. Summary design set-up for Runs 1-56 (in 3 parts). 

17 
18 
Figure 7, part 3. 

19 
Experiments in Homogeneous Materials 

Several sapping runs were conducted in homogeneous sediments for purposes of 

establishing a set of controls to be used for comparison with experiments using layered 

stratigraphy and structural variations. Figure 8 summarizes the design for each of the runs 

done with uniform sediments, which included Runs 1,2,3,23,34, and 35. Runs 1,2, and 3 

all used fine-grained sand extracted from a point bar on the Mississippi River at Grand 

Tower, Illinois. Table 6 provides a summary of the grain-size characteristics of the 

Mississippi River sand as well as the other sediments used in later experiments. Run 23 

used fine sand that was weakly cemented. Runs indicating cemented sand were prepared 

by mixing small concentrations of portland cement (typically e 1 %) with the sand while dry 

in order to provide cohesion and to decrease permeability. The mixture was then wetted to 

just below saturation and allowed to set up for 24 hours prior to the experiment. 

RUN 1 

.. ....... .. .'. ....... ............... ....... 

................. 

RUN 3 

... .: . . ...... ...... e.. 1. ....... ...... ........ .- .. ... ................. 

RUN 23 

EXPLANATION 

[13 .. fine sand 

coarse sand 

SCALE 

(cm) 

200 100 0 

100 

50 

0 

Figure 8. Generalized setup for rus using homogeneous sediment. 

Table 6. Grain Size Data for Flume Sediments 

Size Fraction Fine Sand & Loess Fine Sand Sieved Coarse Sand Coarse ~~ Sand 

ChY 9.03% 

Fine Silt 12.89% 0.54% 1.90% 2.34% 

coarse silt 25.85% 

Fine Sand 28.19% 52.81 % 25.20% 23.62% 

Medium Sand 22.54% 45.08% 34.68% 34.4 1 % 

Coarse Sand 0.45% 0.89% 36.96% 26.38% 

Pebble 0 0 0.84% 12.45% 

20 
Channel Evolution 

1, 

E 

uv 

Lo 

2. !2 

Following establishment of the reservoir head, the saturated zone migrated 

downflume through the sediment wedge during each run until the water table intersected 

either the escarpment at the base of the wedge or the mid-slope region of the wedge, 

whereupon sapping began. In most runs a rather steep hydraulic gradient was observed 

from the reservoir surface to the intersection of the water table and the sediment wedge. 

Although there were slight differences in the rates of valley establishment between runs 

depending upon initial slope and material variations, a general sequence of sapping valley 

evolution was common to each run. The channel evolution sequence was characterized by 

four distinct phases shown schematicaly in Figure 8: 1) Channel initiation; 2) rapid 

adjustment and competition between adjacent channels for available groundwater; 3) channel extension (headcutting) and accelerated groundwater flow convergence; and 4) 

channel stabilization. 

CHANGES IN RATES WITH TIME 

.20 

200 0 3 HRS 

_. 

41 'I 

ADJUSTMENT AND STABILIZATION 

TIME 

Figure 9. (top) Idealized channel evolution phases observed in the flume. 

(bottom) Example of sequence from Run 3. 

21 
Channel initiation occurred by one of two processes depending upon the location 

of the intersection of the water table and the downflume surface of the sediment wedge. In 

most runs, the water table intersected the sediment wedge along its mid-slope region, 

resulting in the development of a linear saturated seepage face or zone that extended across 

the entire width of the wedge and was typically between 5-10 cm in thickness parallel to the 

flume axis. Immediately downslope of the seepage face, emergent water began to form 

numerous micro-rills. Within minutes, several of these rills would become the preferred 

loci of groundwater flow convergence and ultimate locations of sapping valleys within a 

few tens of minutes. Between two and four equally-spaced channels were established 

during the channel initiation phase of each experiment. Between-channel spacing is 

determined by the extent of subsurface recharge feeding each channel consistent with the 

model developed by Dunne (Figure 2). The constancy of spacing between initial channels 

probably reflected an equilibrium adjusted to boundary conditions established by the 

thickness of the wedge, the width of the flume, and the permeability of the sediments. 

During this initiation phase, the dominant rills rapidly downcut and began to extend 

headwardly as groundwater seepage forces lifted grains off the bed and from the base of 

headscarps in the channels, inducing mass wasting of overlying channel walls. Once 

removed from the walls, sediments continued downslope by fluvial processes downstream 

of the sapping zone. 

An alternative mode of channel initiation occurred when the water table intersected 

the nearly-vertical escarpment near the base of the wedge. The height of this escarpment 

varied with the slope of the wedge surface. Excessive pore pressures along this seepage 

zone promoted extensive slumping of the wedge. Initially, slumping resulted in parallel 

slope retreat, but within a few tens of minutes, accelerated slumping at several locations 

across the wedge (near the sites where dominant channels formed when sapping valeys 

were initiated by mid-slope seepage zones) resulted in the establishment of three or four 

dominant chaiiieis. Ciimneis spaced abut 1ni apaii seemed :a refleet qtiExiiiia spacing 

for valleys formed in unconsolidated sediments in the flume. Continued seepage through 

the slump debris removed sediments until incipient sapping channels were formed. Local 

steepening of the hydraulic gradients in the areas of accelerated slumping promoted 

groundwater flow convergence into the channel, resulting in predominant channels. 

Sediment wedges with slopes in excess of 12O typically failed by extensive 

slumping and resulted in the termination of the run. Sapping valleys failed to become 

initiated in runs with initial slopes less than 60 because these slopes were below the 

threshold needed to transport sapped debris from the seepage face by fluvial processes in 

the small channels. 

The second phase of channel evolution was characterized by rapid channel 

adjustment and competition for groundwater between neighboring initial channels. The 

initial channels immediately began to downcut and extend headwardly as groundwater flow 

converged at the channel head accelerating the sapping processes in the manner described 

by Dunne (1980). In every case, at least one of the three or four initial channels had its 

groundwater supply pirated in the subsurface by competition of neighboring channels. 

Ultimately, the pirated channel became inactive and remained unchanged throughout the 

remainder of the run. Successful channels rapidly evolved from narrow V-shaped rills to 

broad U-shaped channels with flat valley floors and amphitheater heads typical of the 

Martian valleys proposed for sapping origin. This phase of development was generally 

complete within one or two hours after channel initiation. 

22 
The third phase of channel evolution was characterized by continued groundwater 

flow convergence and rapid channel extension and enlargement. During the extension 

phase, the active channels enlarged rapidly by incision and headward extension. Incision 

generally proceeded at a linear rate until the channel head reached a zone of equilibrium 

equivalent to the location of the intersection of the water table on the upper reaches of the 

wedge. During the enlargement phase, no tributaries ever formed along downslope reaches 

of the channels. In addition, springs and seepage zones were noticeably absent along 

lower reaches of the channels (Figure 10). All of the active sapping was confined to the 

headward 25% of the channel areas. Some bifurcation was common along the channel 

heads, but rarely persisted to the extent that integrated networks developed. Development 

of integrated networks was inhibited by the relatively low cohesion of the sediments 

necessary to promote effective removal in the scale of our experiments. Slumping of the 

promontories between extending channel branches rapidly destroyed interfluves in most of 

the runs at the heads of any given channel attempting to bifurcate. Theoretical modelling by 

Howard (1988) illustrates how the network would probably have developed, provided 

scaling parameters had not interfered with the experiment. 

UNIFORM FINE SAND 3-LAYER STRATA 

Figure 10. Sapping valleys form from a combination of sapping-mass wasting processes 

operative in a zone near the seepage face. Fluvial processes dominate downstream. 

In the active seepage zone, granular removal of sediment grains was visible as well 

as extensive small-scale slumps. Channel walls remained nearly vertical but were 

extensively scalloped by slumping processes. Slumps were preceeded by the formation of 

small-scale alcoves in upstream channel walls and channel headwalls in the sapping zone. 

Channel extension was episodic. Therefore, in micro-time scales, valley extension was 

likewise episodic. Periods of catastrophic change were follwed by longer periods of slow 

adjustment and material removal. Periods of relative quiesence and granular removal were 

interrupted by catastrophic slumps whose debris would fill channel floors until it could be 

' 

23 
removed by continued seepage and downstream fluvial processes. Howard and McLane 

(1981) described a similar episodic nature to the evolution of sapping escarpments in their 

two-dimensional experiments. During this period of rapid extension and incision, channel 

widening did occur, but it was not prominent. 

Simultaneously, extensive alluvial fans were prograding at the mouths of the 

sapping channels. Although the fans were not specific targets of our investigation, their 

dynamics were intimately invovled with sapping processes operating in upstream reaches 

of the channels. During periods of rapid channel enlargement and high rates of sediment 

production, fans actively prograded. During periods of relative channel stability, fans were 

trenched reflecting diminshed sediment supply from upstream. An interesting footnote 

related to the fans was that the stability of fan surfaces was also observed to be directly tied 

into fluctuations in the reservoir head. Therefore, this indicates that fluctuations in water 

tables, perhaps due to cyclic climatic variations, can ultimately cause thresholds in fan 

incision and accretion to be crossed. This observation has interesting implications to the 

interpretion of downstream sedimentary sequences in sapping-dominated channels and 

even to environmental problems on terrestrial alluvial fans. A special run (Run 56) was 

included at the end of the experiments to document the response of sapping fans to 

temporal flucuations in the water table. 

The final phase of channel evolution is termed channel stabilization.. Stabilization 

is actually a misnomer, however, the rate of channel growth eventually became exceedingly 

slow compared to early phases (See Figure 9) so that it could be considered relatively 

stable. Fanhead trenching typically coincided with the onset of this phase of channel 

growth. Stabilization occurred when channels had extended headward to the point of 

coincidence with the intersection of the slope and the water table. Then, channel incision 

continued, but at a slower pace, while headcutting typically ceased. At the same time, the 

rate of head widening often increased dramatically during this latter phase. Head widening 

was now promoted by the stabiiity of the seepage zone across the wedge. Valleys typically 

developed enlarged amphitheaters at their heads for a period of time. Finalley, even the rate 

of headcutting diminished in runs with uniform sediment. 

Examules from Homogeneous Sediments 

Run 3 best typifies the control runs using homogeneous fine sand and is illustrate 

in Figures 11 and 12. Run 3 had a surface slope of loo and used a constant head of 35 cm 

in the reservoir for 46 hours. Figure 11 shows oblique photographs taken at hours 15 and 

46. Four channels formed during the channel initiation phase during the first two hours. 

Channel incision and rapid headward extension during hours 1-5 was followed by gradual 

incision and enlargement during the stabilization phase between hours 6-46 (Figure 12). 

Abandonment of the third channel from the left occurred between at hour 10. The resulting 

final form was three prominent channels having amphitheater heads and relatively uniform 

channel width along their axes. Valley walls showed prominent slumps and scalloped 

margins indicative of the importance of mass wasting processes triggered by groundwater 

sapping. This run provided a good control for contrasting with later runs in varied 

geologic settings. 

Run 23 provides a comparison of the effects of increased cohesion and decreased 

permeability with the unconsolidated sand used in Run 3. A 0.3% mixture of cement was 

added to the fine sand in Run 23 for greater cohesion. One prominent channel and two 

smaller ones once were formed by groundwater sapping in Run 23, but the general 

characteristics of these channels differed significantly from those formed in Run 3. 

24 
Figure 11. Oblique and overhead photos of Run 3. 

Clockwise from upper left: 1) 20 hrs, 2) 46 hrs, 3) 46 hrs, 4) 20 hrs. 

25 
3UN 3 - GRADIENT (Chan 2) 

HOURS 

- n4 

-4m 

eo 

M 

EIrUN 3 - HEAD WIDiH CK4"EL 

-1 

-1 

r 

ao 

-u 

n 

5 80.' 

z 44.' 

W 

s 48.. 

*4 

Ki 

38 .. 

a SOL 

80 110 140 170 200 230 

Distance Up Channel (cm) 

C 

RUN 3 - HEADCUTTING 

1 CHANNEL 

/' 

/ 

f' -1 

-n 

I 

I 1 

4 Q 24 33 48 b T I rn e (hours) 

- 

4 0 24 33 48 

T i m e (hours) 

Figure 12. Morphodynamics of Run 3 in homogeneous sediments. 

26 
Modifications Using Uniform Sand 

Run 33 provides interesting observations from an experiment done in weaklycemented sand similar to the mixture described for Run 23. These runs were designed to 

see how preformed channels with predetermined junctions would be modified by 

groundwater sapping processes. This scenario may be useful in providing ideas about 

what the effects might be of a climatic change which would cause a runoff-dominated 

valley network to become dominantly affected by sapping processes. A single dendritic 

channel network was carved by hand into the sediment surface as illustrated in Figure 13 

which had three upstream bifurcations established with junction angles commonly found in 

terrestrial runoff networks. Following the construction of the channels, the overhead 

sprayer was used to produce runoff for 2 hours. Runoff had no apparent effects on the 

channels except to continue to enlarge them without significantly altering the junction 

angles. After the episode of rain, sapping commenced and was permitted to occur for 194 

hours. Early in the sapping run, minor extension occurred along the upper reaches of the 

first order branches. Meanwhile, small seepage rills began to develop at locations 

independent from the precut channels by seepage erosion. These seepage channels 

subsequently progressed through the normal sequence of channel development until a 

dominant one between the two pre-cut upper channels joined the trunk of the pre-cut 

system. During the remainder of the run, the sapping valley continued to enlarge and 

remained independent of the network initially cut into the wedge. 

Run 33 illustrated that sapping processes can be significant enough to cause 

significant alterations in the morphology of pre-existing channels and may become useful in 

making interpretations about paleoclimatic flucuations in terrestrial and Martian valleys. 

Baker and Partridge (1986) observed two distinct styles of valleys in dry valley networks, 

termed degraded and pristine. Pristine valleys commonly occupied downstream reaches of 

the same networks that contained degraded valleys, causing Baker and Partridge to 

specil!ate hat he pristice segments may reccrd a rejuvenation cf channel incision. It is 

possible that the pristine valleys, which are typically more sapping-like in appearance, may 

record an alteration in channel morphology caused by the onset of groundwater sapping. 

Run 35 looked at the effects of sapping upon previously established channel 

networks. In this case, two networks were cut into the wedge with parallel drainage 

patterns composed of only two branches (Figure 14). The use of parallel networks 

removes some of the suspicion that the changes observed in Run 33 were due simply to the 

shift from dendritic to parallel drainage that would be expected for any channel on steep 

slopes such as these regardless of the formation by sapping or runoff. The run was 

allowed to proceed for 77 hours and resulted in significant alterations to the original 

channel patterns. This time, no new channels were created, but substantial modifications 

did occur to junction angles and symmetry of the original valley networks as a result of 

sapping. The channels originally oriented such that they could most efficiently capture the 

subsurface water entering from the reservoir became the two prominent channels while 

pirating supplies of groundwater from their adjacent channels. In contrast, the channel 

oriented almost parallel to slope enlarged in a regular manner consistent with the style 

observed in Run 3. The channel on the left, oriented more obliquely to slope, developed a 

distinct asymmetry. 

Sapping modifications caused the oblique channel to extend directly up the dip 

slope toward the reservoir in the direction of preferred hydraulic gradient. This asymmetry 

of channel pattern seems especially common in valley networks influenced stongly by 

sapping due to the strong preference for valleys to extend up dip slopes along preferred 

hydraulic gradients. In virtually all sapping runs, channels showed accelerated slumping 

along the wall oriented up the dip slope, reflecting the uneven entrance of groundwater 

27 
flow into the channels. The direction of asymmetrical extension was controlled by 

structural dip and by groundwater flow net interferences caused by competing valleys. 

Many of the Martian slope valleys along Valles Marineris, for example (Figure 15), have a 

distinct asymmetry which may be related to slope. Studies by Laity and Malin (1985) and 

our studies on the Colorado Plateau also contain numerous examples of asymmetrical 

valley networks in areas of dipping strata strongly influenced by sapping processes. 

Figure 13. Photos from Run 33. 

Clockwise from upper left: 1) initial setup, 2) 40 hrs, 3) 157 hrs, 4) 194 hrs. 

28 
Figure 14. Photos from Run 35. Clockwise from top leftp: 1)1.5 hrs - initially 

following joints, 2) 31 hrs - developing up-dip sapping; 3) - 56 hrs - up-dip sapping 

deviates from joint trends, 4) 77 hrs - continues up-dip sapping. 

29 
Fi-gure 15. Examples of asymmetry common in Mars s!ope valleys along Valles 

iilarineris. Note the pronounced extension of channels toward the upper left. 

Rainfall Runoff Networks in Uniform Sediments 

In several runs, rainfall was applied to the sediment wedges in an attempt to create 

channel networks for comparison to sapping valleys. Rainfall channels typically developed 

wide, straight valleys parallel to slope with little channel identification. Head areas of 

rainfall channels were indistinct and blended into the wedge slope in sharp contrast to the 

deeply-incised amphitheater heads characteristics of the valleys formed by sapping in the 

same sediments. 

Phillips and Schumm (1987) performed a series of experiments creating drainage 

networks in unconsolidated sediments by runoff processes. They showed that runoff 

networks on a wedge of similar size to ours developed well-integrated branching networks 

with substantially high drainage densities. In addition they found that with increasing 

slope, the drainage pattern shifted from dendritic to parallel at slopes less than 20 

concurrent with a decrease in junction angles from approximately 600 to approximately 

400. Therefore, we would expect runoff channels on our wedges with significantly higher 

slopes to be parallel and have even lower junction angles than those reported by Phillips 

and Schumm (1987). These observations argue strongly that the higher junction angles 

sometimes observed in our sapping runs were due to groundwater influence and extension 

along inhomogeneities in the sediments. Likewise, the high junction angles of the 

Hawaiian valleys and the Colorado Plateau valleys assumed to be influenced by sapping 

(see later summary sections) reflect structural control under the influence of groundwater 

sapping. 
Exberiments With Lavered Strahrrrmhy 

..... .... 

A series of experiments were designed using dipping layers with varying 

permeability to simulate the kind of geological setting that occurs in the gently-dipping 

strata of the southern Colorado Plateau. Earlier research by Laity and Malin (1985) and the 

recent field conference on groundwater spping (Howard, Kochel, and Holt, 1988) held in 

the Colorado Plateau have shown that an abundance of sapping phenomena have been 

involved with the evolution of channels in that region. The effects of slope in our 

experiments with layered stratigraphy appeared to be complex. Figure 16 shows a 

summary of the types of experimental setups used in the layered runs. Run 3 can be used 

as the homogeneous control to be compared to Runs 8,9,11,25,26,27,28,30, and 33. 

I RUN 14 A I RUN 28 A l///-&pl .... '. ,:.:. 

..... ........ * ._.,. ...... . ............... 

I RUN 25 I RUN33 

I 

RUN 26 A I EXPLANATION 

fine sand 

coarse sand 

Figure 16. Summary of design set-ups for runs using layered stratigraphy. 

Channels produced in runs with layered stratigraphy followed the regular scenario 

of channel evolution outlined in the section on homogeneous sediments. However, layered 

runs exhibited channels with considerably less development during equivalent time periods 

than either the experiments with uniform stratigraphy or the set of runs to be discussed later 

using high-level aquifers. This indicates the important effect that changing hydraulic 

characteristics of the host sediments can have upon the morphology of the resulting sapping 

valleys. In most of the runs with layered sediments, channel width did not increase 

appreciably in the upchannel direction, but remained relatively constant or even tapered 

slightly. Part of the explanation for the lower rate of head widening may be the limiting 

recharge area available to the growing sapping channel heads imposed by the twodimensional nature of permeable units. The added recharge from below the sapping head 

was reduced in these runs by the presence of relative aquicludes below permeable units in 

contrast to the uninhibited three-dimensional recharge areas around the heads of valleys 

formed in homogeneous sediments. In the latter case, the only limiting factor would have 

been the thickness of the entire sediment wedge, rather than the thickness of a given 

permeable stratum during the experimetns with layered sedments. 

31 
The depth of sapping canyons appears to have been directly controlled by the 

thickness of the sediment in the upper layers of the runs. In situations where there is a 

more permeable upper layer (weakly cemented) over a less permeable base (strong cement 

or loess mixture), the basal layer acts as a base level control on incision. Thick, cemented 

upper layers prevented good channel development in the experiments because channels 

were clogged by massive slumps eroded from valley walls. Coarse slump block debris 

from the resistant caprock in some of the runs could not be transported and formed 

boulders of cemented sand which armored sapping channels. This limited channel 

development in some cases. Less cemented upper strata were readily eroded by fluvial 

processes downstream from the sapping face. 

The width of sapping channels varied considerably with the thickness of the strata 

and with cohesion (Compare Figure 11 (Run 3) with Figure 17 (Runs 14 and 27)). 

Channels were considerably wider in less cohesive sediments (Run 3) where lateral 

migration of streams downslope from the active sapping face was extensive. Cohesion 

limited the rate of lateral cutting by retarding the rate of channel wall slumping, resulting in 

Figure 17. Variations in channel development depending on intersection of permeable 

strata with wedge slope. Left) Run 14,58 hrs - shows minor channels because permeable 

layer did not intersect surface. Right) Run 27,47 hrs - shows deep channels because 

permeable beds intersected slope surface. 

Variations in the dip of the strata appeared to be more important in affecting channel 

development than surface slope. Runs 14 and 27 contained layered strata with markedly 

different surface and dip slopes. The 90 slope in Run 14 should have resulted in rapid 

channel formation. However, only small channels formed directly above the toe of the 

coarse layer. Slumping occurred downslope from the zone of seepage (Figure 17). The 

stratigraphy in Run 14 was parallel to the surface slope and no layering was exposed on the 

seepage face. Most of the groundwater discharge through the coarse layer either flowed 

along the flume floor to. induce slumping or emerged directly above the toe of the coarse 

layer, taking the shortest route through the fine layer. 
The surface slope of Run 27 was only 309 well below the threshold of transport observed for the homogeneous sand runs, but this run experienced significant channel 

development. Channels developed because the coarse, permeable layer was exposed on the 

face of the low scarp at the wedge toeslope. Sapping rapidly produced small slumps which 

extended headward into channels (Figure 17). The coarse sand used in the more permeable 

bed armored the channels, which inhibited incision as the run continued. Therefore, these 

channels were not incised as deeply as those in homogeneous sediments. The plot of 

headcutting shows (Figure 18) where channel 1 quickly developed while other channels 

failed to undergo the rapid extension phase due to piracy until channel 1 approached the 

stabilization phase. 

10 a? 

5 

0 

HEAD WIDTH - RUN 27 

.............. \ 

2 13 36 40.5 46 61 S6 

nme (hours) -. 

1 13 36 40.5 46 61 .. 86 

Time (hours) 

CZ 

36.7 

30.9 

25.0 

4 

GRADIENT - RUN 27 - CH 1 

f I 35 65 15 125 155 185 

.--- ....... -- sa..- I-- u-3.S .... - ,-.,.) 

Figure 18. Channel morphodynamics for Run 27. 

Note how channel (lower left graph) developed before other channels. After left channel 

stabilized, the other channels then permitted discharge to channels 2-4. 
*RICINAI; PAGE: 1s ** QUALrm 

Tributarv Asmmetrv and DiD Sloues in Layered Sediments 

As was shown in the modified homogeneous sediment experiment in Figures 11 

and 12 (Runs 34,35), tributaries developed preferentially along the sides of channels 

oriented oblique to wedge surface slopes in a manner extending directly up-dip toward the 

reservoir. This created a pronounced asymmetry to the overall drainage network. Laity 

and Malin (1985) drew attention to the role of structure in controlling the pattern of channel 

networks developed by sapping in the Navajo Sandstone of the Colorado Plateau. They 

noted that on a regional scale the tributaries to the Escalante River in southern Utah were 

asymmetrically distributed about the main channel. Tributaries were more frequent and 

more incised on the up-dip sides of the channel, reflecting the importance of groundwater 

flow parallel to dip in channel development influenced by sapping processes. 

Run 18 also provided an example of the preferential asymmetry of channels 

developed in sapping systems along regional dip-slopes (Figure 19). Note that the right 

(up-dip) walls of the channel indicated by the arrow are scalloped by sapping alcoves while 

the down-dip slopes (left) are smooth. Similar asymmetry is also visible along valleys 

formed on slopes of Kohala volcano (Kochel and Piper, 1986) and in areas of the Colorado 

Plateau where sapping processes are active along dip slopes (Kochel and Phillips, 

1987,1988). Asymmetrical valleys are also common along Martian slopes as exemplified 

along Valles Marineris in Figure 15. 

Figure 19. Run 18,19 hrs. Note the example of asymmetric channel development. 

Second channel from right is extending headward along its right side. Left side of this 

channel is inactive due to upslope groundwater piracy by the large channel on the left which 

was first to tap into the perched high-level aquifer. 
Figure 20. Photos from Run 30. 

Clockwise from upper left: 1) 18 hrs - mass wasting along sapping channel walls, 2) 32 

hrs - note limited channel development compared to Run 3,3) 56 hrs, 4) overhead view 

showing amphitheater head with slump-dominated morphology 

35 
ExamDle Using Layered StratimaDhv 

Run 30 exemplified a successful run using layered stratigraphy (Figures 20,21). 

In contrast to Run 3 (uniform sand), it required about 4 hours to complete the phase of 

channel initiation in layered sediments, which was followed by a lengthy phase of channel 

adjustment which extended for 32-48 hours, depending upon which channel was being 

observed. Channel stabilization was reached after 48 hours and continued until the run was 

terminated at hour 78. Note the considerable irregularity in channel form compared to 

previous runs in less consolidated sediment as well as the abundance of channel wall 

alcoves in Run 30. Alcoves seemed to be preferentially developed in layered sediments 

with varying permeability. The marked contrasts in vertical permeability apparently permit 

groundwater to become localized along contacts between the units and concentrated flow 

occurs laterally, provided the contact is within the saturated zone. This situation is similar 

to the observations discussed later in the summary of field studies in the Colorado Plateau 

and in the section describing alcoving experiments. 

-\ 

GRADIENT - RUN 30 (CH 1) 

69 95 :19 I25 140 155 

Dtsrcnce ii. 3; 

-~ 

HEAD CUTTING - RUN 30 

0 2 4 11 18 24 40 56 66 78 

Time (hours) 

GRADIENT - RUN 30 (CH 2) c';rpl :;D 

_ 

w 

2o I 10 I 

I 

__ 4 hrt 

.--. 11 hrs 

.__ 18 hri 

___._ 24 hrn 

_._ 32 hra 

___ 40 hrs 

____ 48 hra 

- bG hrl 

-_ 66 hi# 

I ---- 78 hr' 

0- 

80 85 110 12s 

Distance (cm) 

Figure 21. Channel morphodynamics for Run 30. 

36 

ORPGINAE PAGE IS 

OE POOR QUALITY 
Figure 21 shows that headcutting was most rapid followed by incision during the 

early phases of Run 30. In the late hours of the run, the dominant changes were head 

widening as the channels cut down to a coarse, permeable layer. Channel 1 cut further ' upslope than channel 2, but both were rapidly extending until hour 18. After hour 18, the 

dominant process was head widening facilitated by slumping above the sapping faces. 

Photographs (Figure 20) show repeated slump blocks appearing in the channels and their 

removal by seepage flows and downstream fluvial activity. Alcoves remained a prominent 

morphologic feature of valley walls in the areas of active sapping and persisted along the 

walls even after the zone of active sapping migrated upslope with channel headcutting. The 

persistence of relict alcoves on downstream valley walls argues for the relative minor 

influence of lateral migration and fluvial activity in widening downstream areas of sapping 

valleys. 

ExDeriments Using: Joints 

A series of five runs, Runs 8,9,10,11, and 31, tested the effects of joint patterns 

installed into layered stratigraphy on the evolution of channels by groundwater sapping. 

There is considerable evidence from field studies in the Colorado Plateau that groundwater 

movement in consolidated rocks is often preferred and accelerated along zones of structural 

weakness such as faults and joints. The objective of these experiements was to create 

linear avenues of increased permeability with respect to the host strata in order to mimick 

joints. 

In general, main channel trends followed joint patterns and their tributaries formed 

along secondary joints. Channels in Run 11 followed the joints exclusively (Figure 22). 

During the first few hours, seepage occurred along the slope over a large range of 

elevations, but exclusively within the joints. Two main channels formed and elniarged 

hwrs cf the ns Secame it reached zm intersesthn G€ J""" inints fiirst, ax! niratpd r---- water from the 

neighboring channel. In all of the runs, the predominant effects of the joints were observed 

in the upper reaches of the channel networks. In many cases, excessive fluvial activity in 

downstream reaches masked any influence of joints during later phases of the runs. 

Observations in the Colorado Plateau and on Mars images indicate that most of the effects 

of jointing are also best observed in the channel head regions. 

. headward along the joint pattern. The left channel (Figure 22) was favored during the latter 

In Runs 8,9, and 10, the channels did not follow the joints exclusively, but were 

strongly influenced by their presence. Run 8 began with a period of parallel scarp retreat 

by slumping at the toe of the slope. An indentation appeared along the scarp at the right by 

hour 30 (Figure 22) which extended to a joint intersection. By hour 9, this had grown into 

a large channel with a bifurcated head. Bifurcation was attributed to groundwater inflow 

from the two diverging joints upslope from the intersection. Early development in Run 9 

followed the joint pattern (Figure 22) until major joint intersections were encountered. 

Then, the left channel widened rapidly along a major joint creating the diagonal channel. 

The right channel bifurcated at a joint intersection and followed two diverging joints as it 

cut headwardly, which is well illustrated in Figure 22. The rate of headcutting in the left 

channel (Channel 2) was rapid during the first 19 hours as the diagonal channel was 

forming. The rate of growth of Channel 2 then slowed as Channels 4 and 5 enlarged (right 

bifurcation). Apparently, Channels 4 and 5 pirated more water from the neighboring 

channels after reaching the joint intersection on the right. 

37 
Figure 22. Photos from several joint runs. 

Clockwise from upper left: 1) Run 8 - 4 hrs, 2) Run 9 - 11 hrs, 3) Run 9 - 31 hrs, 4) Run 

10 - 24.5 hrs, 5) Run 11 - 20 hrs, 6) Run 11 - 57 hrs. 

38 
ORIGNAL PAGE IS 

OF POOR QUALITY 

Figure 23. Effects of structural control at the heads of slope valleys along Valles 

Marineris, Mars. Valleys are preferentially extending headward along grabens. 

39 
i19A15 

519A35 52w 

519A16 

FRACTURE SYSTEMS 

(locations on Fig.1) 

(n = 2901 % I % * 

3AO I 

n 1519AzI 519A02 

ENTIRE 

AREA 

OUTFLOW 

CHANNELS 

- + 

SAPPING 

CHANNELS 

lnmI121 

GRA8ENS 

ln.130) 

# 

WINKLE 

RIDGES 

6 I2 I8 

f 

Figure 24. Sapping modified margins of western Kasei Vallis, outflow channel on Mars. 

Sapping has extended channel widths considerably by following structural trends. Note 

correspondence of structural features with channels (from Kochel and Burgess, 1983). 
In summary, experiments with linear zones of increased permeability suggest that if 

significant joints are present, sapping valleys will preferentially extend along these avenues 

of increased groundwater discharge. The degree of influence that joints will have upon 

channel location and morphology probably depends upon the relative differences between 

permeabilities of the joints and the host rock. As this difference increases, the influence of 

jointing should become more pronounced. Runs 1 1 and 3 1 had the greatest permeability 

contrasts between the joints and the layers and channels in those runs exhibited the greatest 

adherence to the joint patterns. Channels in Runs 8,9 and 10 exhibited some joint control, 

but channels still developed independent of the joints in several areas because the 

permeability of the sediments was still fairly high relative to the joints. 

Rainfall was applied to some of the joint runs after sapping experiments were 

terminated to observe the influence of runoff on patterns adjusted to joint patterns. The 

onset of runoff resulted in significant adjustments to the sapping valleys. Runoff channels 

tended to adhere to the controls of the surface topographic slope, which in many cases was 

unrelated to the orientation of channels developed oblique to slope by sapping along the 

joints. The most important effect of rainfall was to increase discharge through the former 

sapping valleys which helped to remove slump debris which had accumulated in the 

channels from sapping erosion. Hence, in terrestrial composite channels, the processes of 

sapping and runoff may work in concert to erode channels. Only in areas where sapping is 

exceedingly prominent with respect to runoff will the channels probably acquire 

characteristics distinctive of sapping valleys. 

Martian sapping valleys display numerous examples of structural control in their 

valley patterns. Figure 23 shows headward extension of slope valleys along Valles 

Marineris controlled by the influence of major grabens. Kochel and Capar (1983) showed 

close correspondence of structural features and sapping channels throughout the Valles 

Marineris region of Mars. Kochel and Burgess (1983) observed similar features along the 

wdis of major iviartian outrlow channeis that have been modified by post-flood widening 

processes (Figure 24). 

Experiments With Structurallv Controlled Perched High-Level Aauifers 

Our field investigations in large, amphitheater-headed valleys on Hawaii provided 

evidence that they are influenced by large volumes of groundwater fed into their heads from 

dike-impounded aquifers perched near the summits of volcanoes (Kochel and Piper, 1986). 

Neighboring valleys which fail to intersect these aquifers are considerably less incised and 

show characteristics typical of parallel drainage networks formed by runoff. A series of 

experiments were designed to observe the effects of discharge from high-level aquifers into 

sapping valley systems and to contrast this with rates of valley formation in the same 

sediments formed exclusively by runoff. Runs 7,18,22,24,32,45, and 49 were designed 

to view the effects of high-level aquifers (Figure 25). Several of these runs used a coarsegrained wedge (Le. Run 7) in the upper slope as a surrogate for a high-level aquifer, while 

other runs used cemented sand dikes (i.e. Run 18) to perch water in the upper slope region 

until dikes were breached. 

\ 

41 
RUN 11 

=.. _ . -. - . - . - 

SCALE ._._._. . - . - . - . - . - . - . 

(cm) 

200 100 

EXPLANATION 

Q fine sand Cemented 

coarse sand weak 

fine sand 8. loess strong 

100 

50 

0 

Figure 25. Summary of set-ups for stratigraphic high-level aquifer runs. 

High-ievei Gravei Aauifer Runs 

The resdts of Run 7 typify the experiments with high-level aquifers (Figures 

26,27). Run 7 contained a wedge of coarse, more permeable sand in the rear of the 

sapping wedge sandwiched between two layers of fine sand of the type used in Run 3. 

The coarse wedge extended down-flume only 1.5m from the headboard and did not 

intersect the sediment surface. The effects of the accelerated discharge through this aquifer 

became increasingly apparent as incision and channel development progressed through the 

run. Once channels incised deeply enough to tap into the perched aquifer, their rates of 

growth accelerated. Figure 26 shows that piracy of one channel occurred and its lower 

reaches were eroded during expansion of the neighboring channels to the extent that the 

inactive channel appears to be a tributary to one of the major valleys. Compare the final 

stage of the channels with that from Run 3 in homogeneous sediments. The channel head 

for the third channel from the left (channel 2) is almost three times as wide as the head area 

in Run 3. Figure 27 illustrates that head width increased dramatically between hours 12- 

18, corresponding to the time when channel incision intersected the buried aquifer. The 

rate of headcutting slowed markedly during this same period as the channel concentrated on 

lateral widening along the seepage face of the aquifer. Downcutting and headcutting rates 

were significantly reduced after the aquifer was tapped. The lateral growth of valley head 

regions resulted in the near merger of two channels which caused the intervening channel to 

be abandoned due to subsurface piracy. This situation is very similar to the scenario we 

proposed to explain the merger of Waipio and Honokane Valleys on Kohala volcano on 

Hawaii and the intervening piracy of channels (Kochel and Piper, 1986). 

42 
ORIGNA~; PAGE xs 

OE POOR QUALITY 

Figure 26. Photos for Run 7 with high-level stratigraphic aquifer. 

Clockwise from top left: 1).5 hrs - aquifer is untapped, 2) 18 hrs - 2 channels tap aquifer, 

3) 42 hrs - extensive head widening at aquifer face, 4) 42 hrs - overhead view. See also 

Figure 19 for 19 hrs. 

43 
ZUN 7 - GRADIENT (Chan 1) 

68 

80 100 120 IS0 180 210 a Distance Up Channel (cm) 

RUN 7 - HEADCUTTING w 1 CWNEL 

-1 Id 

c b 7 10 17 23 42 

T I m e (hours) 

RUN 7 - HEAD WIDTH 

Figure 27. Channel morphodynamics for Run 7. Note accelerated growth after perched 

aquifer is tapped at hour 17 (from Kochel and Piper, 1986). 

High-Level Aauifers Tm~ounded bv Dikes 

Run 18 (Figure 28,29) provides a good example of the experiment using cemented 

dikes to trap high-level aquifers in a manner similar to the dike-impounded aquifers in 

Hawaiian volcanoes. Homogeneous fine sand was used throughout the sapping wedge. A 

relatively impermeable dike was placed midway through the wedge, dipping toward the 

headboard at about 700. Five channels initially formed within the first three hours of Run 

18, four of which remained active throughout the remainder of the run. Channels incised 

and enlarged steadily, but more slowly than typical for runs in fine sand during the first 18 

hours due to the decreased groundwater discharge downslope from the dike. Between 

hours 18-25 channel 3 incised through the dike, permitting additional groundwater to enter 

the channel. Rates of head widening then increased dramatically following the breach of 

the dike for channel 3. Rapid head widening in channel 3 soon pirated groundwater from 

neighboring channels (completed by hour 50), resulting in their abandonment. At hour 68, 

abandonment is well illustrated in Figure 28. Channel 6 also breached the dike at hour 62 

and then rapidly widened its headwad region as well. 

The results of Runs 7 and 18 illustrate how sapping channels respond to the effects 

of structural or stratigraphic inhomogeneities in the subsurface which provide sudden 

increases in groundwater discharge into the channels. Tapping the high-level aquifers acted 

to rejuvenate the channels and they shifted from a stabilized condition into a new phase of 

rapid adjustment and enlargement (Figure 30). The final channel form was a much 

enlarged valley with a bulbous upper reach and well-developed amphitheater head. 

44 
Figure 28. Photos from Run 18 with dike perched aquifer. Clockwise from upper left: 1) 

4 hrs - dike is untapped, channel development is slow, 2) 18 hrs - dike is tapped by central 

channel which rapidly expands, 3) 42 hrs - left (central) channel has rapidly expanded 

while channel in foreground is still attempting to cut through dike, 4) 68 hrs - central 

channel is very enlarged, right channel has just tapped aquifer and begins to enlarge. 
?UN 18 - GRADIENT (Ch 

sol. I - 

80 110 140 170 200 250, 

Distance Up Channel (cm, 

RUN 18 - HEADCUTTING JJO h 

U 

280 W 

0 

c 

CY 

v) 230 U 

.- 

n 

U 

3 

0 100 x 

a 

I 

1 so 

T I m e (hours) 

RUN 18 - HEAD WIDTH 

.................... 'A. ..... ......... :.-.::/ .. , 

-:::-.:..*:..-: ::----. 

2 10 27 40 74 100 

T i m e (hours) I 

Figure 29. Channel morphodynamics for Run 18. 

the diked aquifer is tapped at 18 hrs (from Kwhei and Piper, 1986). 

Note rapid growth of channels once 

Runs that used two dikes (such as Run 45) had sLmi!ar results to Run 18 except that 

the channel experienced two distinct episodes of channel rejuvenation in a manner 

consistent with that described for Run 18. The end result was a planform for the valleys 

that had distinct buldges in valley width immediately upstream from both of the dikes 

(Figure 3 1 =photo). 

Finally, several dike runs were made using a split flume design which permitted the 

introduction of surface runoff to both sides of the flume while restricting groundwater 

sapping to only one side. Run 49 serves as a good example of this experimental design. 

During the first 5 hours channels developed solely from runoff introduced by a trickling 

pipe placed at the upper end of the sediment wedge on both sides of the wood divider 

between the two flume halves. Following the initial period of channel development, the 

reservoir was raised and one side was allowed to experience sapping while both sides 

continued to receive runoff from the pipes. The channel on the sapping half rapidly 

enlarged following the introduction of sapping flow and downcut while the runoff valley 

continued to grow at an exceedingly slow rate. The limited discharge into the runoff 

channel caused this system to remain transport-limited and the valley terminated in a 

narrow, tapered upstream end, not much changed from its configuration established near 

the outset of the run. In contrast, the sapping channel enlarged by significant headcutting, 

developed a broad, flat-floored valley, and terminated upslope in a broad, amphitheater 

head. These experiments provide conceptual support for the model we proposed to explain 

the large Hawaiian valleys as being strongly influenced by sapping from the dikeimpounded aquifers while their neighboring smaller valleys develop only in response to 

runoff. 
n' 

z 

0 i y S'I'AIJILIZA'I'ION - --- AFI'ER 

TAPPING AQUIFER 

----------- 

ORIGINAL STABILIZATION TREND / 

TIME 

Figure 30. (top) Idealized channel evolution showing adjustment to new equilibrium 

conditions after threshold is crossed when aquifer is tapped. (bottom) Example of situation 

from Run 32 which had two dike-trapped aquifers. 

Figure 31. Overhead photo from Run 45 showing bulges of channels immediately 

upslope of two dikes whose outline can be seen running east-west across photo. 

47 
AlcovinP ExDerimentS 

Runs 39-44,46,48, and 50-54 were designed to determine if alcoves, like those that 

are ubiquituous throughout the walls of canyons in the Colorado Plateau in permeable 

sandstones, could be developed in an experimental setting. The goals of this work were to 

try to determine the influence of variables such as stratum thickness, dip, and joints upon 

the development of alcoves by groundwater sapping. The results of these experiments had 

mixed success. Water was introduced into the system by a spraying system of perforated 

hoses mounted to a board above the flume to mimick precipitation-recharged systems on 

the Colorado Plateau. Stratified wedges were set up having a vertical cliff at the downflume edge which exposed the internal stratigraphy. Alcoves were formed in the flume 

experiments using layers of weakly cemented sand dipping about 20 alternating with thin 

interbeds of heavily cemented sand which served as barriers to groundwater flow. These 

alcoves had morphologies very similar to those formed in the Navajo Sanstone of the 

Colorado Plateau. However, most of the alcoves formed only at the basal contact between 

a permeable unit and impermeable base stratum because of the difficulty in establishing 

lateral groundwater flow above the saturated zone. 

Seepage did occur immediately above higher aquicludes, but its discharge was too 

insignificant to overcome the resisting forces of surface tension and cohesion in the grains 

to effectively erode material from the escarpments. Experiments with stacked impermeable 

units to mimick interdune aquicludes found in the Navajo Sandstone showed progressive 

wetting of the beds immediately above the aquicludes during experiments even in the 

absence of significant alcove production. 

SUMMARY OF FLUm EXPERiMEhiS 

Experihental model!ing pvided a convenient way of making direct observations 

of the process and morphology of valleys formed by groundwater sapping. It is very 

difficult to resolve questions relating to the formative processes of valleys in the field many 

times because of the complexity of terrestrial channels. Terrestrial channels are usually 

composite channels, but may be dominated by sapping or runoff processes. In the lab, we 

can evaluate the evolution of valleys by each process by controlling the conditions of the 

experiment. Controlled laboratory settings are also required to observe evolutionary 

processes over short periods of time. We could not mimic all of the attributes of field 

settings due to scaling problems such as: 1) the important role of chemical weathering and 

removal of sapped debris; 2) actual rock strength; and 3) sediment size scaling. However, 

the observations made in the flume provided an excellent conceptual framework for us to 

apply to the interpetation of terrestrial analog examples and the interpretation of Martian 

valley networks. 

These experiments proved extremely useful in constructing conceptual models of 

how sapping-dominated valleys evolve. In particular, the experiments point to the episodic 

nature of changes at the sapping head related to slumping. Similar importance of slumping 

has been observed in terrestrial settings such as the Hawaiian valleys (Scott and Street, 

1976) which will be discussed in the next section. In addition, the experiment 

demonstrated the overwhelming role groundwater piracy has upon valley development and 

form. Piracy was shown to be directly controlled by the structure of the underlying 

geologic materials. 

48 
Our experiments sucessfully demonstrated the following features which have been 

suggested to be important in the evolution of channels on the Colorado Plateau: 1) joint 

control of groundwater flow and subsequent orientation of channels with predominant 

effects seen in the headward reaches of the channels; 2) the importance of groundwatrer 

piracy; 3) the importance of stratigraphy in controlling the location of seepage faces in 

layered rocks; and 4) has shown how impermeable strata can interact with the geometry of 

permeable strata as important controls on the dimensions of channels developed by 

groundwater sapping. 

These kinds of modelling experiments are particularly good for: 1) testing 

concepts; 2) developing a suite of distinctive morphologies and morphometries indicative 

of sapping; 3) helping to relate processes to morphology; and 4) providing data necessary 

to assess the relative importance of runoff, sapping, and mass wasting processes on 

channel development. 

The results of these experiments, concluded in late March 1988, are being 

assembled and will be prepared for publication in the near future. In addition, we hope to 

collaborate with Alan Howard at Virginia on some of the publication efforts. 

TERRESTRIAL ANALOG STUDIES 

OF VALLEYS INFLUENCED BY SAPPING 

Before reliable interpretations can be made concerning the genesis of Martian valley 

networks by assigning them to sapping or runoff systems, an integrated comparative 

investigation of sapping and runoff dominated terrestrial aiiaiogs must be coriipleted to 

construct models for distinguishing between the channel types. This was the primary 

r\bjective r\f the second phase of our research - to develop a suite of geomorphic crtiteria 

from terrestrial environments that could be used in remote sensing obsrvations of Mars to 

distinguish between channels formed by runoff or sapping. 

The areas selected for this phase of terrestrial analog studies were: 1) large valleys 

on several Hawaiian Islands; and 2) selected portions of the Colorado Plateau based on 

reconnaissance studies during the NASA Field Conference (Howard, Kochel, and Holt, 

1988). 

HAWAIIAN VALLEYS 

The islands of Hawaii, Maui, and Molokai contain many examples of extremely 

large, deep, flat-floored valleys having amphitheater heads which are markedly different 

than their neighboring valleys. The large Hawaiian valleys occupy wet, windward slopes 

of Hawaiian Islands and pose a stark contrast to the insignificant valleys which occur on 

the dry, leeward volcanic slopes. However, the differences between the valleys can not be 

fully ascribed to climatic variations because there are small, less-incised valleys are also 

found on windward slopes nestled in among the large valleys. We used a combination of 

field reconnaisance studies and morphometric analyses of basin characteristics made from 

aerial photographs and topographic maps to comare these valley types on the windward 

slopes of Kohala (Hawaii), Maui, and Molokai. The results of this investigation are 

summarized in detail by Kochel and Piper (1986) and by a forthcoming thesis by Piper 

(1988). Therefore, we will only present a summary in this report. 

49 
Field Reconnaissance 

Field reconnaissance was used to determine the influence of sapping on the small 

and large valleys along the windward slopes of Kohala (Hawaii), Maui, and Molokai 

(Figure 32). Large springs emerging from the base of the amphitheater headwalls of the 

large valleys were observed, even in the absence of surface runoff over the headwalls. 

Further inspection revealed the presence of numerous dikes, as mapped by Stearns (1946) 

in valley head areas. These observations, and the lack of basal springs in the smaller 

valleys, led us to conclude that the large valleys were strongly dominated by groundwater 

sapping while the smaller valleys were formed by runoff processes alone in accord with a 

similar model proposed by Stearns (1966) and summarized in Figure 33. Additional field 

observations showed that the sapping-dominated valleys (See Figure 34 were typified by: 

1) abrupt valley wall and floor junctions; 2) abundant evidence of mass wasting along 

valley walls; and by 3) large springs emerging from alcoves at the base of valley-head 

amphitheaters. In areas where waterfalls plunged over valley walls, insignificant plunge 

pools were observed, indicating that the dominant mode of headward retreat was from 

sapping erosion undercutting valley headwalls and subsequent mass wasting of the 

overhanging basalt. Hawaiian sapping-dominated valleys typically widen toward their 

heads in contrast to the runoff-dominated valleys which taper upslope (Figure 35). 

Sapping valleys have incised to sea level and enter the ocean with broad mouths while 

runoff valleys enter the sea as spectacular waterfalls from hanging valleys (Figure 35). The 

enlarged sapping valley heads at the elevation of the dike swarms which contain the highlevel aquifers have widened to the point that merger of Waipio and Honokane valleys on 

Kohala has effectively pirated groundwater previous bound for intervening valleys. The 

experiments discussed previously using dikes and stratigraphic high-level aquifers 

produced find morphologies very similar to the basins observed along the eastern slopes of 

Kohala volcano (See for example, Runs 7 and 18). 

50 
/ 

VTS IN CM (Islands not to scale) 

1 fho w 1 Lo 158O 

67 LAROE VALLEY AREAS 

4 PREVAILING TRADE WINDS 

(1.3)ZAGE IN MILLION YEARS B.P. 

V MAJOR VOLCANIC PEAKS 

22O N 

. 

- 21° 

KAHOOLAWE 

/KOHALA AREA 

SCALE 

Figure 32. Index map and climate data for Hawaiian analog study sites. 

Figure 33. Model for high-level aquifers trapped by central volcanic dike swarms. 

Runoff channels successful in tapping dikes will enlarge rapidly due to sapping influence. 

51 
PACIFIC OCEAN 

a OUATERNARV ALLUVIUM (IlurlaI.~~ailal) 

HAW1 VOLCANICS (andeslllcl 

POLOLU VOLCANICS (basaltic) 

CONES. DOMES 

9 MAJOR BASALTIC DIKES AND INTRUSIONS 

:* CANYON RIMS IN LAROE VALLEYS 

PACIFIC OCEAN 

SCALE 

.- 5 10 (km) 

PA HAWAII 

a 

Figure 34. Drainage and geologic map for Kohala Volcano on Hawaii. Note enlarged 

valleys affected by sapping and the coincidence of their heads with exposure of dikes. 

52 
Figure 35. Photos showing contrasting niorphology of sapping and runoff valleys on 

Kohala. Clockwise from upper left: 1) tapered heads on runoff valleys (Fig. 34, valleys 

R,Q), 2) amphitheater heads of tributaries to sapping valley b in Fig. 34,3) hanging 

mouths of runoff valleys, 4) enonnous mouths of sapping valleys cut to sea level. 

53 
Remote Studies and Comparative Analysis 

Figure 36. Examples of PCA from Kohala showing clear separation of sapping and 

runoff valleys (from Kochel and Piper, 1986). , 

I 

54 

Following the categorization of valleys as sapping-dominated vs. runoff-dominated 

from the field work, we gathered data on the morphometric characteristics of the valleys 

from analysis of aerial photographs and topographic maps. Six parameters included in the 

analysis proved to be statistically significant in distinguishing the valley types using 

principal components analysis (PCA) (Table 7). Figure 36 shows that PCA was successful 

in statistically distinguishing sapping and runoff valleys on the basis of morphometric 

parameters visible in remote imagery. 

PC1 KOHALA PC2 KOHALA 

70 

c 

0 

a 50 

40 

Figure 36. Examples of PCA from Kohala showing clear separation of sapping and 

runoff valleys (from Kochel and Piper, 1986). 

54 

i 

PC3 KOHALA 
Table 7. Geomorphic Parameters Used in PCA 

Parameter Explanation 

K 

'IDD 

MJA 

F1 

R2 

BCR 

basin shape using lemniscate 

total drainage density 

mean junction angle for basin 

first order stream frequency 

cross-axial relief at mid basin 

basin aredcanyon area 

Table 8. Co mDm 'son of SaDjing and Runoff Vallevs in Hawaii 

ffrom Kochel and PiDer. 19861 

RUNOFF vs. SAPPING VALLEYS in HAWAII 

(from Kochel & Piper 1986) 

PARAMETER RUNOFF-DOMINATED 

basin shape very elongate 

head termination tapered, gradual 

channel trend uniform 

pattern parallel 

junction angle low (30'-45') downstream tributaries frequent 

relief low 

drainage density high drainage symmetry symmetrical 

basin area/ canyon area very high 

SAPPING-DOMI JATEI) 

light-bulb shape (wide head) 

amphitheater, abrupt 

variable 

dendritic 

higher (55O-65O) 

rare 

high 

low 

asymmetrical 

low 

55 
Table 8 summarizes the morphologic and morphometric differences between 

sapping-dominated and runoff-dominated valleys on Molokai and Hawaii. The major 

distinguishing characteristics of mature sapping valleys are their blunt amphitheater heads, 

the increasing valley width usptream, the lack of parallel drainage, the small basin 

wedcanyon area, and the asymmetry of drainage observed in tributary patterns. The 

degree of valley widening depends on structure, stratigraphy, and time as shown by the 

sapping box experiments. This study also demonstrated that there are a number of 

morphometric criteria of the type that can be identified from remote imagery which can be 

used to statistically distinguish between process origin of valley types within a selected area 

of relatively consistent geologic conditions. These kinds of parameters will be applied to 

the analysis of Martian channels in ongoing research to help interpret channels genesis. 

Vallev Evolution on Hawaii 

Many of the Kohala valleys exhibit characteristics of both valley styles in different 

portions of their drainage basin. Sapping canyons extend in a headward direction after 

beginning at midslope at the major dike swarms. Drainage characteristics upstream from 

the sapping headwall exhibit characteristics of runoff valleys, Le., parallel drainage, 

shallow incision, and tapered heads. Eventually, headward retreat of the sapping face 

results in channel extension to basin divides. This "mature" morphology is visible in the 

older sapping valleys that characterize the north side of Molokai (Figure 37). Figure 38 

summarizes an evolutionary scheme proposed for the Hawaiian sapping valleys. Hawaiian 

valleys originate from runoff processes as elongated valleys with high drainage density and 

parallel drainage (Stage 1). Slope valleys on Mauna Loa, Mama Kea, and the west side of 

Kohala typify this "youthful" stage on Hawaii. Continued valley incision eventually taps 

into perched dike water (Stage 2) which marks the initiation of the dominance of 

groundwater sapping processes in these valleys. Subsequent development (Stage 3) is 

characterized by headward extension of the sapping face and deep valley incision. 

Honokane Vaiiey on Hawaii and Halawa Vaiiey on Moioliai ;are examples of S:age 3. I 

-. PACIFIC OCEAN --t - *' 

KAMALO 

Figure 37. Drainage map of eastern Molokai. Extensive sapping valleys are shaded. 

56 
PROCESS 

DRAINAGE 

DENSITY 

VALLEY EVOLUTION 

12 3 4 5 

TiRilE 

Figure 38. Evolution of sapping valleys and changing dominance of runoff and sapping 

processes (from Kochel and Piper, 1986). 

Continued headward growth and valley widening results in a transition from 

runoff-dominance to sapping-dominance concurrent with downstream decrease in drainage 

density, abstraction of downstream tributaries due to subsurface piracy, and a shift toward 

dendritic or rectangular drainage patterns. Valley extension by sapping eventually proceeds 

to the basin divides (Stage 4). At this point, subsurface piracy causes extensive head 

widening, and "mature" sapping valleys are created, which many times behead subsurface 

and surface drainage to neighboring valleys. Waipio and Honokane Valleys on Hawaii 

have merged on Kohala and the effect has been a marked reduction in flow and valley 

growth to the valleys in between these two. 

Many sapping valleys on Molokai have proceeded to stages 4 and 5. These valleys 

are increasingly beingly influenced by surface processes, which produces a greater 

drainage density and more dendritic drainage pattern. Generally, in terrestrial runoff 

systems, drainage density is a measure of erodibility and climate in a watershed. On 

Hawaii, drainage density is more controlled by the predominance of sapping or runoff 

processes during basin evolution. As more precipitation is delivered to the mainstream by 

groundwater discharge, drainage density decreases. In late stage valley evolution, 

57 
precipitation is again dominantly delivered to the channels by runoff, hence, drainage 

density again starts to increase. Valley evolution ultimately proceeds until adjacent valleys 

merge as divides are breached. The resulting degraded and complex drainage system is 

characterized by the basins seen on the older islands of Oahu and Kauai. More refinement 

of this evolutionary model will be presented in a master's thesis by Piper (1988). This 

evolution described for Hawaiian sapping valleys has many parallels to the channel 

evolutionary sequence described for the flume studies done with uniform sediments, 

layered sediments, and experiments with high-level aquifers. 

SAPPING VALLEYS ON THE COLORADO PLATEAU 

During the past year we began to study potential sapping valleys as terestrial 

analogs in the Colorado Plateau, following in the lead of Laity and Malin (1985) and the 

suggestions of our Field Conference in 1985 (Howard and Kochel, 1988; Howard, 

Kochel, and Holt, 1988). Again, a combination of field reconnaisance for purposes of 

establishing sapping or runoff dominance (as well as collecting other related data on the 

morphology of sapping-related features) and remote studies was used as a framework of 

study with two major objectives in mind: 1) to continue to refine the construction of a suite 

of geomorphic criteria useful in distinguishing sapping-dominated from runoff-dominated 

valleys with the addition of data from sedimentary layered terrain; and 2) determining the 

hierarchy of geologic controls on sapping valley development with the goal of being able to 

use valley morphology as a means of interpreting underlying geological conditions. 

N 

r 

Utah region 

.A I 

Indian Reservation % 

1.- 0 1nn km 

Figure 39. Index map of study areas on the Colorado Plateau. 

58 
The focus of the Colorado Plateau investigations was in two areas that contained 

prolific examples of sapping valleys interspersed with examples of runoff valleys in similar 

and different lithologies. The areas chosen were the Tsegi Canyon-Shonto Plateau region 

of northern Arizona and the lower portion of Lake Powell in Utah (Figure 39). The 

attention was focused upon valleys incised into the porous and permeable Navajo 

Sanstone. The Navajo Formation has ideal characteristics for the interception, 

transmission, and discharge of groundwater. Precipitation rapidly infiltrates into the unit 

from abundant surface potholes and moves downward until it reaches a less permeable 

barrier (Figure 40). Concentration of groundwater flow along impermeable barriers results 

in lateral flow that produces zonal seeps and results in the erosion of the poorly-cemented 

sandstone along canyon walls (Figure 41). This sapping process eventually undermines 

the material above, causing collapse and the production of alcoves along with eventual 

valley extension. 

,surface Dotholes 

Figure 40. Schematic of infiltrating precipitation 

through potholes into Navajo Sandstone. At the 

boundary flow begins to move laterally down dip. 

Figure 41. Schematic showing the 

localization of groundwater flow 

along impermeable boundaries. 

Seepage erosion results in extensive 

alcove production. 

59 
Distinguishing Between Sauuing and Runoff Valleys 

The dominance of sapping vs. runoff processes was determined from field 

reconnaissance. Sapping dominated valleys were observed to have large numbers of active 

seeps and associated alcoves in their headward regions and along tributaries. Runoff 

valleys had a paucity of these features and were presumed to be affected only by 

intermittent runoff events. A subset of 19 basins were selected from a variety of lithologies 

in the area of Shonto Arizona (Figure 42) for morphometric analyses based on the 

measurements from aerial photographs and topographic maps. Figure 43 shows the 

variety of drainage networks developed across the region which reflect both the differences 

in underlying lithology. The significant parameters and comparisons between sapping and 

runoff valleys are shown in Table 9. Sapping dominated valleys have higher junction 

angles, are more elongate, have lower drainage densities, have higher relief, and have 

greater basin area in canyon than their neighboring runoff dominated counterparts. The 

same comparisons hold true whether basins in similar or different lithologies are 

considered. 

'. 

Figure 42. Index map showing basins used in morphometric analyses reported in Table 9. 

60 ORIGINAL PAGE IS 

OB POOR QUALITY 
rnparison of Sappine and Runoff Vallevs in Shonto Area of Arizona 

I and Phw 19871 

Table 9. Co .. 

DI<AINAGE BASIN COMI’AIUSONS 

niean 21.1 73.2 24.9 45.8 9.9 4.2 297 

std dcv 17.8 3.4 12.8 12.4 1.4 0.7 79 

RUNOFF BASlNS 

(n = 8) 

mean 48.5 52.1 17.4 17.0 12.2 5.7 150 

std dev 38.1 6.9 8.1 12.6 5.3 4.4 38.6 

JCT =junction angle 

DD = drainage density 

CAN = 9% basin in canyon 

F1 = frrst order stream frequency 

Figure 43. Aerial photo of Shonto study area. Tsegi Canyon and Navao Creek Canyon 

sapping valley systems are deeply incised. Other areas contain runoff valleys. Note the 

view of a portion of Black Mesa at lower right which is underlain by other rock types. 

61 
Additional differences between sapping and runoff dominated valleys can be seen in 

the comparison of longitudinal valley profiles (figure 44). Sapping valleys have abrupt 

head terminations and are generally more gentle throughout the remainder of their profiles 

downstream from the sapping face. Runoff valleys display smooth, concave-upward 

profiles typical of rivers in rocks with relatively unifomi resistance. 

Table 10 shows an inventory of the kinds of parameters found to be significant in 

distinquishing between sapping and runoff valleys from the combined field and 

experimental studies. The parameters which appear in the imagery category can be directly 

applied to the interpretation of valley networks visible in Viking imagery on Mars in 

attempts to interpret valley genesis. For areas of Mars where adequate topographic data 

becomes available, some of the topographic variables can be added into the analysis. This 

will be the topic of our research effort during the current year. 

Selected Streaiii l'rol'iles 

SAPPING 

r 9000 

Keet See1 

6000 

SU 30 10 

RUNOFF 

Geslii r 

70UU 

60UO 

so00 

90 70 50 30 1u 

15 

i 

I', Yellow Wnter r8OOO " 

Uegasliibito 

.009 

N 

8000 I 

7000 

6000 ti 

E SUO0 ,. 

c 

'I' 150 130 110 90 70 50 30 IO 

Distalice in reel x IOU0 

Figure 44. Comparison of the longitudinal profiles of sapping and runoff valleys on the 

Colorado Plateau. 

62 
Table 10. Geomorphic Criteria for Distinguishing Valley Origins 

P- SAPPING RUNOFF Field Observation Topograp hic Map Aerial Imagery 

Lithology & Structure 

lithology permcable variable X X X 

facies Variations X 

directional permeability will orient variable X 

sedimentary structures major effect variable X 

joints, faults - orientation major effect variable X X 

channel head density low high X X 

degree of extension toaquifer variable X X 

rim/perimetcrballcy arm high low X 

drainage density low high X 

basin ardcanyon area low high 

axial valley gradient abrupt smooth X 

valley length, width bulbshape tapered X 

seep discharge high low X 

alluvial deposits complex fluvial seds X 

downstream discharge decreases increases X 

Mass wasting & Walls 

freq., distrib. rockfalls nearseeps random X X 

relative age of deposits cyclic clim. cyclic clim. X 

heraction with alluvium complex X 

Headscarp Morphology 

Valley Morphology 

X 

channel, valley x-section abrupt, U smooth, V X X 

Hydrology 

X 

X 

X 

Geological Controis on SauDinP-Dominated Vaiievs 

Apart from distinguishing &tween sapping and runoff valleys, the second goal of 

our research on the Colorado Plateau was to extend the investigations of Laity and Malin 

(1985) to develop a better understanding of the geological controls on sapping valley 

development and how these controls are reflected in the morphologic expression of the 

valleys. Various geological parameters were investigated to determine their effect upon 

valleys and develop a hierarchy of importance of these controls (Kochel and Phillips, 1987; 

1988). Additional research is currently in progress on this topic which will be summarized 

in future publications and in a nearly completed master's thesis by Phillips (1988). 

First Order Controls - Diu 

Many factors influence groundwater sapping processes in sedimentary rocks. 

These can be organized into a hierarchy based upon their scale of influence on valley 

geometry (Figure 45). Major canyons and intermediate-sized tributary systems appear to 

be dominantly influenced by the dip of the strata because of preferred access to 

groundwater flowing down-dip. The experiments in layered strata nicely illustrated the 

preference of channels to extend directly up the dip of the strata regardless of the orginial 

configuration of the channel network. Sapping dominated valleys on the Colorado Pateau 

are typically asymmetrical, with their longest tributaries occurring along their up-dip sides 

of the master channel (Figure 46). In contrast, short valleys on the down-dip sides of the 

main channel will probably remain small because they receive only localized groundwater 

recharge (Figure 47). A survey of active seeps in alcoves around the Shonto field area and 

the Lake Powell area demonstrated that most of the active seeps are located in alcoves 

oriented down-dip (Figure 48). In areas of constant dip, active alcoves are uniformly 

63 
oriented in that direction. In contrast, in areas of complex and variable dip, active seeps are 

likewise oriented in a multitude of directions. Therefore, on a limited basis, the orientation 

of active alcoves can be used to infer structure. 

Controls on Sapping Valley Morphology 

, I Second Order 

II First Order JOlNTS 

DIP I I - major tributary axis 

- minor tributary axes II - vallev head extension -dominant valley axes 

' I - dornhant valley axes? 

Third Order 

SEDIMENTARY 

STRUCTURES 

local alcove dev. 

and activity 

2ndary alcoves 

- iiiterdune deps. 

- bedset boundary 

- cross-bedding & 

directional 

permeability 

- primary alcoving at Navajo/Kayenta boundary 

Figure 45. Hierarchy of geological controls on valleys influenced by groundwater sapping 

in the Colorado Plateau. 

Figure 46. Aerial photo illustrating asymmetrical sapping valleys in Tsegi Canyon. Note 

the extension up-dip (to upper right). Down-dip tributaries are much smaller. 
Tsegi Canyon 

Figure 47. Schematic of groundwater recharge and asymmetrical valley development for 

Tsegi channels seen in Figure 46. 

Laity and Malin (1985) showed that the asymmetry of valleys on opposite sides of 

the Escalante River in Utah was due to variations in dip. Large, sapping channels formed 

on up-dip sides of the valley, while smaller, less-developed runoff valleys formed on the 

opposite side in opposition to the regional dip of the strata. Figure 49 shows symmetrical 

sapping valleys disposed about a major channel tributary to the San Juan River. This 

pattern is indicative of a syncline with the main channel along the fold axis, which happens 

to !x t!~e case. Therefore, in mmy cases, the asyz~~etr;l of sapping vd!ey lnletwzrks CLY be 

used to infer geologic structure, chiefly dip, underlying the valley. 

Figi Ire 49. Synclinal axis running down axis of the main channel of the large sappii 

valley shows doubly extending sapping tributaries up each flank of the fold. 

65 
Escalante River Area 

ALCOVES JOINTS CANYONS 

mi tnacttve 

0 acttve 

Lower Colorado River Area 

N ALCOVES JOINTS CANYONS 

n 321 n 149 n a5 

inactive 

active 

Figure 48. Relation of strata dip and orientation of active seeps and alcoves. top) 

Constant dip with focused active alcoves in same orientation. bottom) Variable dip and 

similar variability in orientation of active seeps and alcoves. 

Second Order Controls - Joints and Fault5 

Our experiments with joint simulations demonstrated a strong preference for the 

localization of groundwater flow in layered sediments along joints, The influence of joints 

in the experimental systems was particularly prominent along the upper reaches of the 

valley network where actvie sapping was occurring. Joints provide local zones of 

increased permeability, localizing groundwater flow in consolidated rocks and helping to 

accelerate sapping processes. These effects are particularly pronounced in the Colorado 

Plateau (Figure 50) valleys underlain by the Navajo Sandstone. Effects of joints are 

particularly prominent along valley heads and tributaries (Figure 50). Note the remarkable 

similarity to valley heads on Mars along Valles Marineris that are extending along grabens 

parallel to the main canyon Figure 5 1). The prominence of joints helps to explain the high 

junction angles characteristic of valley networks strongly influenced by sapping processes. 

66 
ORIGINAL PAGE IS 

OF POOR QUALIT'II; 

In Tsegi Canyon, the combination of joint orientation and strata dip can explain 

most of the variation in valley orientation. One of the prominent joint directions coincides 

with the dip orientation which is parallel to the major tributaries, while the other prominent 

joint trend is parallel to the major Tsegi valley (See Figure 46). 

Figure 50. Influence of joints on valley head areas of sapping channels in Tsegi Canyon 

(left) and Nokai Canyon (right). 

Figure 51. Joint influence on valley heads (right) along Valles Marineris, Mars. 

Compare with Figure 50. 

67 
Third Order Controls - Sedimentarv Structures 

Variations in the internal sedimentological charcteristics of rocks such as the Navajo 

Sandstone also affect the morphology of sapping-dominated valleys, but on more localized 

scales (Kochel and Riley, 1988). Aeolianites such as the Navajo have numerous internal 

bounding surfaces across which there are major permeability contrasts that strongly 

influence the pattern of valley development by groundwater sapping (Figure 52). 

@ Bounding Surface Rank 

'$ Foreaet Laminae 

\ Bounding Surface 

EXPLANATION 

Interdune Deposits 

Dune Deposits Parallel 

Dune Deposits Perpendlcular 

to Strlke of Dune ' 

to Strlke 01 Dune 

-c Preferred Fluid Mlgratlon 

Figure 52. Schematic of major bounding surfaces in aeolianites like the Navajo Sanstone 

along which groundwater can be concentrated. 

Interdune facies are one example of these variations (Figure 53). Interdune 

deposits are typically formed of impermeable carbonate which forms major ledges along 

canyon walls cut into the Navajo. Major seeps and alcoves are localized above these 

interdune deposits, resulting in the formation of multi-level or tiered alcoves. Laity and 

Malin (1985) observed that the largest alcoves in the Navajo are located along the boundary 

between it and the underlying Kayenta Formation. However, many regions have very 

large alcoves developed at many levels controlled by the stratigraphic location of interdune 

deposits (Figure 55). The limited lateral extent of interdune facies also acts as a major 

control on the lateral dimensions of alcoves. Alcoves have been observed to totally pinch 

out at the edge of interdune layers becaue there is no longer an impermeable boundary 

along which to localize and concentrate seepage. Petrographic studies are underway to 

document differences in relative permeability of the Navajo Sandstone at various locations 

with respect to alcoves. 

Another locally important permeability contrast in aeolian sandstones like the 

Navajo are the boundaries of major coss-bed sets, which are typically several tens of 

meters thick. Fine-grained sediments often occur at these boundaries and localize seepage 

along their margins. 

68 
-small alcov 

interdune de 

big alcove 

re 

posits 

Figure 53. Schematic of localization of seeps above interdune deposits. This results in 

multi-level alcoves in the Navajo Sandstone. 

ALCOVE LOCATION 

Navajo Creek 

n = 353 

no 58.1 x -I 

41.4 X 

501 :'i 10 

0 

Naval0 Navajo - 

Frn. Kayenta 

0 

- 

0.8 x 

Kayenla - Wlnaale 

contact conimct 

Tsegi Canyon 

11-222 

87.4 "LJ -- 185 

-eo 

- so 

- 40 

- 30 

fnmp obrorvmllona 

llold obwervatlona - 20 

12.8 x 

- 10 

0 Navslo Navajo - Frn. Kayenla 

contact 

Tsegi Canyon 

11-222 

87.4 "LJ 185 

eo 

so 

40 

30 

fnmp llold obrorvmllona obwervatlona 12.8 x I 20 

Navslo Navajo - " 

Frn. Kayenla 

contact 

Figure 54. Stratigraphic location of alcoves surveyed in Shonto area. Many occur within 

the Navajo due to sedimentological controls. 

69 
Finally, the dip directions of major cross-beds can influence groundwater flow. 

Lateral permeabilities are much higher than vertical permeabilities across cross-bed 

laminations (Figure 55). Studies of directional permeability for petroleum reservoirs have 

shown drastic variations dependent upon cross-bed orientation (Lindquist, 1983). This 

directional permeability can be important as a major control when there is regional 

consistency of cross-bed direction which does occur in the Navajo outcrop region of the 

study area (Figure 56) (Marzolf, 1983). We recently completed several experiments in the 

sapping flume designed to observe the effect of cross-bed orientation on groundwater flow. 

The results of these runs indicated that groundwater discharge was significantly greater 

parallel to the cross-bedding as opposed to across the cross-beds. Numerous examples of 

seeps were observed in the field which also supported the model represented in Figure 57. 

Sedimentary features appear to locally important in modifying sapping valley 

orientation and morphology, provided the features are large enough and of great enough 

regional extent. Most of the features described here would apply to most rocks originally 

formed in aeolian depositional environments and probably to many other rocks having 

sedimentary origin. 

100 OlllCCTlON 

caw 1 

I- I 10 

I' OUT PUNTER ncm. WOWNO .1 

Figure 55. Variation in directional permeability in aeolianites due to facies variations and 

cross-bed orientations (from Lindquist, 1983). 

70 
NAVAJO SANDSTONE 

SOUTHEASTERN UTAH 

STRATIGRAPHIC SECTIONS 

DEWEY ERIDQE 

CAPITAL REEF 

Figure 56. 

HART DRAW 

IOOm lo 

TABUL4R PLANAR 

WEDOE PLANAR 

CROSS-STRATA 

e CROSS-STRATA 

T~~OUGHCROSS-STRATA 

HORIZONTAL STRATA 

[3 STRUCTURELESS 

@ CR03S-STRATA 

CROSS-STRATA 

RECUMBENT 

UNDULATORY 

CONTORTED STRATA 

CARBONATE 

COME RlDQE 

(modllled from Marroll 1~83) 

EXAMPLES OF 

PALEOCURRENT DATA 

DEWEY ERIDQE 

0 Om 

HART DRAW 

0 

ESCALANTE RIVER 

0 Om 

Regional constancy in dip of major cross-bedded sets within the Navajo 

Sziidstane in the sout!elern Cdoiiido Plateaid. 

rq$ \.\\\\\\ ,\,\ \\\\\\,\ 

Figure 57. Directional permeability observed in the field sites by active seeps when threedimensional outcrops with dune cross-bedding were located. 

71 
SUMMARY OF TERRESTRIAL ANALOG STUDIES AND 

APPLICATIONS TO MARS 

Studies from diverse geological terrains hosting sapping-dominated valleys on 

Earth provide essential ground truth information necessary for constructing models for 

interpreting channel and valley network genesis on Mars. Figure 58 shows the how the 

various kinds of data discussed in our research report will provide a data base for 

interpeting Martian channel networks during the following year. The integration of the 

flume studies, the field observations, and the remote studies of terrestrial field sites has 

provided a set of geomorphic criteria visible in remote photographic imagery which will 

calibrate morphometric studies and interpretation of Martian valleys genesis. 

ield Studies 

I 

sedimentary 

Experimental I 

Studies 

m 

I 

/\ 

(qualitative 

modeling) 

0\ 

Figure 58. Schematic showing the flow of thought used in the combined terrestrial analog 

and flume experiments toward building a model for application to the interpretation of Mars 

valley networks. 

The experimental studies provide a base of observations that have been exceedingly 

useful in understanding how channels are initiated, how they develop, and how they evolve 

under the dominance of groundwater sapping processes. Unlike runoff channels, sapping 

valleys can be distinctly separated into two systems on the basis of process dominance. 

Headward reaches undergoing active sapping are dominated by groundwater emergence 

and granular removal by seepage forces as well as catastrophic mass movements of an 

episodic nature. As a result, wall and channel head morphologies become amphitheatershaped, scalloped, and very steep. Downstream reaches are dominated by fluvial 

processes, but they often retain relict sapping features for considerable time periods. 

The specific size and extent of sapping valleys is controlled by the complex 

interaction of stratigraphy which controls the access of groundwater supplies available for 

flow convergence in channel heads. Valley form typically shows complicated patterns of 

asymmetry influenced by structure and examples of groundwater piracy are numerous. 

72 
Most of the features and processes observed directly in the experiments could be 

applied to field situations in the terrestrial analog studies. Together, these studies enable us 

to construct a set of geomorphic criteria which can be recognized on aerial imagery, and, is 

useful in distinguishing sapping and runoff channels. These data will be used in mapping 

channels on Mars and will help interpret the nature of underlying materials. 

In addition, these studies suggest that in some cases, significant interpretations may 

be possible about the nature of underlying geologic structure and lithology, if not in an 

absolute manner, perhaps in a relative contrast to adjacent regions of Mars. In particular, 

attention will be given to the classification of Martian valley types according to genesis as 

sapping vs. runoff dominated. Additional attention this year will be given to regions with 

abundant channels such as some of the highland regions of great antiquity, the Valles 

Marineris area, and some of the volcanic slope channels to investigate what inferences can 

be made about the underlying geologic features. 

73 
REFERENCES 

Baker, V.R., 1973, Paleohydrology and sedimentology of Lake Missoula flooding in 

eastern Washington: Geol. Soc. America Spec. Paper 144,79. 

Baker, V. R., 1978, The Spokane flood controversy and the Martian outflow channels: 

Science, v. 202, p. 1249-1256. 

Baker, V.R., 1982, The Channels of Mars: Austin, Univ. Texas Press, 198p. 

Baker, V.R., and Kochel, R.C., 1978a, Morphological mapping of Martian outflow 

channels: Porc. 9th Lunar & Planet. Sci. Conf., p. 3181-3192. 

Baker, V.R., and Kochel, R.C., 1978b, Morphometry of streamlined forms in terrestrial 

and Martian channels: Proc. 9th Lunar & Planet. Sci. Conf., p. 3193-3203. 

Baker, V.R., and Kochel, R.C., 1979, Martian channel morphology: Maja and Kasei 

Vallis: J. Geophys. Res., v. 84, p. 7961-7983. 

Baker, V.R., and Milton, D.J., 1974, Erosion by catastrophic floods on Mars and Earth: 

Ic~~us, V. 23, p. 27-41. 

Baker, V.R., and Partridge, J.B., 1986, Small Martian valleys: Pristine and degraded 

morphology: J. Geophys. Res., v. 91, p. 3561-3572. 

Dunne, T., 1980, Formation and controls on channel networks: Prog. in Phys. Geogr., v. 

4, p. 211-239. 

Dunne, T., 1988 in press, Hydrology and mechanics of erosion by subsurface flow: in 

Higgins. C.G., and Coates, P.R., eds., Grc;i;iid.iY.aier Geornorphoiogy: Geol. 

SOC. America Spec. Paper. 

Higghs, C.G., 1982, Drainage systems developed by sapping on Earth and Mars: 

Geology, v. 10, p. 147-152. 

Higgins, C.G., 1984, piping and sapping: development of landforms by groundwater 

outflow: in LaFleur, R.G., ed., Groundwater as a geomorphic agent: London, 

Allen & Unwin, p. 18-58. 

Higgins, C.G., and Coates, D.R., eds., 1988 in press, Groundwater geomorphology: 

Geol. Soc. America Spec. Paper. 

Howard, A.D., 1988 in press, Groundwater sapping experiments and modelling at the 

University of Virginia: in, Howard, A.D., Kochel, R.C., and Holt, H., eds., 

Proceedings and Field Guide for the NASA Groundwater Sapping Conference, 

NASA, Washington, p. 132- 160. , 

Howard, A.D., and Kochel, R.C., 1988 in press, Introduction to cuesta landforms and 

sapping processes on the Colorado Plateau: in, Howard, A.D., Kochel, R.C., and 

Holt, H., eds., Proceedings and Field Gudie for the NASA Groundwater Sapping 

Conference, NASA, Washington, p, 1-91. 

74 
I 

Howard, A.D., and McLane, C., 1981, Groundwater sapping in sediments: theory and 

experiments: NASA Tech. Memo. 84211, p. 283-285. 

Howard, A.D., Kochel, R.C., and Holt, H., eds., 1988 in press, Proceedings and Field 

Guide for the NASA Groundwater Sapping Conference, NASA, Washington, 

208p. 

Kochel, R.C., and Baker, V.R., 1981, Modification of escarpments along channels and 

plateaus on Mars: NASA Tech. Memo. 8421 1, p. 321-323. 

Kochel, R.C., and Burgess, C.M., 1983, Structural control of geomorphic features in the 

Kasei Valleis region of Mars: NASA Tech. Memo. 85127, p. 288-290. 

Kochel, R.C., and Capar, A.P., 1983, Structural control of sapping valley networks along 

Valles Marineris, Mars: NASA Tech. Memo. 85127, p. 295-297. 

Kochel, R.C., Baker, V.R., Simmons, D.W., and Lis, C.J., 1983, Surface channel 

networks developed by groundwater sapping in a laboratory model: Analog to 

sapping channels on Mars: NASA Tech. Memo. 85127, p. 227-229. 

Kochel, R.C., and Peake, R.T., 1984, Quantification of waste morphology in Martian 

Kochel, R.C., and Piper, J.F., 1986, Morphology of large valleys on Hawaii: Evidence 

fretted terrain: J. Geophys. Res., v. 89, p. C336-C350. 

for groundwater sapping and comparisons with Martian valleys: J. Geophys. Res., 

V. 91, p. E175-E192. 

Kochel, R.C., and Phillips, M.A., 1987, Geological controls on the morphology of v&---- I-CI.. cys li~luenced by gowidwater sapping on the CdoiabO Platexi; Geol. Soc. 

America Abs. w. Frog., v. 19, no. 7, p. 731. 

on valleys influenced by groundwater sapping, southern Colorado Plateau, Arizona 

and Utah: Abs. 19th Lunar Planet. Sci. Conf., p. 615-616. 

Kochel, R.C., and Riley, G.W., 1988 in press, Sedimentologic and stratigraphic 

variauons in sandstones of the Colorado Plateau and their implications for 

groundwater sapping: in, Howard, A.D., Kochel, R.C., and Holt, H., eds, 

Proceedings and Field Guide for the NASA Groundwater Sapping Conference, 

NASA, Washington, p. 102-1 15. 

Kochel, R.C., Simmons, D.W., and Piper, J.F., 1988 in press, Groundwater sapping 

expeiments in weakly consolidated layered sediments: A qualitative summary: in 

Howard, A.D., Kochel, R.C., and Holt, H., eds, Proceedings and Field Guide for 

the NASA Groundwater Sapping Conference, NASA, Washington, p. 161-183. 

Kochel, R.C., and Phillips, M.A., 1988, Preliminary investigation of geological controls 

Kochel, R.C., Howard, A.D., and McLane, C., 1985, Channel networks developed by 

groundwater sapping in fine-grained sediments: analogs to some Martian valleys: 

in, Woldenberg, M., ed., Models in Geomorphology: Boston, Allen & Unwin, p. 

3 13-341. 

75 

Mars Channel Working Group, 1983, Channels and valleys on Mars: Geol. SOC. America 

Bull, V. 94, p. 1035-1054. 
Marzolf, J.E., 1983, Changing wind and hydrologic regimes during deposition of the 

Navajo and Aztec sandstones, Jurassic?, Southwestern United States: in 

Brookfield, M.E., and Ahlbrandt, T.S., eds., Eolian Sediments and Processes: 

Amsterdam, Elsevier, p. 635-660. 

Masursky, H., Boyce, J.M., Dial, A.L., Schaber, G.G., and Strobell, M.E., 1977, 

Classification and time of formation of Martian channels based on Viking data: J. 

Geophys. Res., v. 82, p. 4016-4038. 

Geophys. Res., v. 78, p. 4037-4047. 

in layered sediments: experiments and field analog: M.S. Thesis, Southern Illinois 

Univ., Carbondale. 

Milton, D.J., 1973, Water and processes of degradation in the Martian landscape: J. 

Phillips, M.A., 1988 in prep., Geological controls on groundwater sapping valleys formed 

Phillips, L., and Schumm, S.A., 1987, Effect of regional slope on drainage networks: 

Geology, v. 15, p. 813-816. 

Pillans, B., 1985, Drainage initiation by subsurface flow in South Taranaki, New Zealand: 

Geology, v. 13, p. 262-265. 

Piper, J.F., 1988 in prep., Development and character of large valleys on Hawaii formed 

with the influence of groundwater sapping: Experiments and remote studies and 

comparisons to Mars: M.S. thesis, Southern Illinois Univ., Carbondale. 

Pollack, J.B., 1979, Climate change on the terrestrial planets: Icarus: v. 37, p. 479-553. 

Sagan, C., Toon, OB., and Gierasch, P.J., 1973, Ciimatic change on -Mars: Science: v. 

181, p. 1045-1049. 

Schumm, S.A., and Phillips, L., 1986, Composite channels on the Canterbury Plain, New 

Zealand: a Martian analog: Geology, v. 14, p. 326-329. 

Schumm, S.A., Mosley, M.P., and Weaver, W.E., 1987, Experimental fluvial 

geomorphology: New York, John Wiley, 413p. 

Scott, G.A.J., and Street, J.M., 1976, The role of chemical weathering in the formation of 

Hawaiian amphitheater-headed valleys: Zit. fur. Geomorph., v. 20, p. 17 1-186. 

Sharp, R.P., 1973, Mars: fretted and chaotic terrain: J. Geophys. Res., v. 78, p. 4073- 

4083. 

Sharp, R.P., and Malin, M.C., 1975, Channels on Mars: Geol. SOC. America Bull., v. 86, 

p. 593-609. 

Squyres, S.W., 1984, The history of water on Mars: Ann. Rev. Earth & Plan. Sci., v. 12, 

Steams, H.T., 1946, Geology of the Hawaiian Island, Hawaii Div. Hydrog. Bull. 8, 106 

Stearns, H.T., 1966, Geology of the State of Hawaii, Pacific Books, Palo Alto, 266p. 

p. 83-106. 

P. 

Toon, O.B., Pollack, J.B., Ward, W., Burns, J.A., and Bilski, K.., 1980, The 

astronomical theory of climate change on Mars: Icarus, v. 44, p. 552-607. 

76