WATERFALLS AND SPRINGS ARE BOTH KNOWN TO UNDERCUT THE BASE OF VALLEY HEADSCARPS ON EARTH, FACILITATING MASS WASTING AND EXTENDING THEATER-HEADED FLUVIAL VALLEYS INTO LAYERED ROCK. AS MANY THEATER-HEADED VALLEYS OCCUR ON MARS, THESE END-MEMBER HYPOTHESES HAVE MOTIVATED CONSIDERABLE ATTENTION AND DEBATE WITHIN THE PLANETARY COMMUNITY, BUT THEY HAVE NEVER BEEN SUBJECTED TO QUANTITATIVE FIELD TESTING ON THE EARTH, EITHER INDIVIDUALLY OR IN COMBINATION. THE RELATIVE IMPORTANCE OF RUNOFF IS STILL NUMERICALLY UNCONSTRAINED, DESPITE THE FACTS THAT RAINFALL IS NEARLY UBIQUITOUS, FLASH FLOODS ARE COMMON IN CANYONS, AND DISCHARGE IS A MEASURABLE VARIABLE WITH A WELL-ESTABLISHED AND QUANTIFIABLE GEOMORPHIC EFFECT. A LITHOLOGICAL AND HYDROLOGICAL MODEL HAS BEEN DEVISED WHEREBY SEEPAGE PROCESSES COULD DOMINATE EROSION OF A VALLEY, BUT IT REMAINS UNKNOWN WHETHER FEATURES OF THIS MODEL APPLY UNIVERSALLY OR ARE EVEN COMMON IN THEATER-HEADED VALLEYS, AS FEW OF THEM HAVE BEEN STUDIED IN DETAIL. THE GOAL OF THIS PROJECT IS TO DETERMINE WHETHER OVERLAND FLOW PLAYS A SIGNIFICANT OR EVEN DOMINANT ROLE IN FORMING THEATERHEADED VALLEYS IN SEVERAL REGIONS OF THE UNITED STATES, OR IF DIRECT RUNOFF IS ESSENTIALLY NEGLIGIBLE IN SOME AREAS, LEAVING A GROUNDWATER SAPPING ORIGIN AS A UNIQUE INTERPRETATION. OUR FIRST OBJECTIVE IS TO EXAMINE THE DIVERSITY OF LITHOLOGY, STRATIGRAPHY, STRUCTURE, HYDROLOGY, WEATHERING ENVIRONMENTS, AND BASE-LEVEL CONTROL IN THEATER-HEADED VALLEYS, IN ORDER TO ESTABLISH THE CONTROLLING VARIABLES ON VALLEY MORPHOLOGY. OBJECTIVE 2 IS TO CONSTRAIN THE DOMINANT DISCHARGE, WHICH IS RESPONSIBLE FOR CHANNEL DIMENSIONS AND MUCH OF THE SEDIMENT TRANSPORT IN FLUVIAL CHANNELS, USING WELL ESTABLISHED PALEOFLOW TECHNIQUES. OBJECTIVE 3 SEEKS TO DETERMINE THE WEATHERING PROCESSES AND RATES IN THESE VALLEYS, TO EVALUATE WHETHER OVERLAND FLOW OR GROUNDWATER SAPPING COULD REASONABLY BE RESPONSIBLE FOR PRODUCING MOST OF THE MOBILE SEDIMENT. THESE THREE OBJECTIVES WILL PROVIDE THE FIRST QUANTITATIVE CONSTRAINTS ON EROSION PROCESSES IN THEATER-HEADED VALLEYS, WHICH ARE A WELL ESTABLISHED ANALOG TO MARTIAN VALLEY NETWORKS. THIS PROJECT IS WELL SUITED TO THE MFRP GUIDELINES OF (I) THEORETICAL AND EXPERIMENTAL STUDIES, INCLUDING LABORATORY STUDIES OF ANALOG MATERIALS, TO INVESTIGATE THE COUPLED ATMOSPHERIC AND GEOLOGICAL SYSTEMS ON MARS; AND (II) QUANTITATIVE TERRESTRIAL FIELD EXPERIMENTS THAT IMPROVE UNDERSTANDING OF THE IN SITU MEASUREMENTS THAT HAVE BEEN OR THAT WILL BE MADE ON MARS. THE PROJECT WILL ADDRESS THE NASA RESEARCH GOAL EXPRESSED IN THE MEPAG DOCUMENT: GOAL III: DETERMINE THE EVOLUTION OF THE SURFACE AND INTERIOR OF MARS; OBJECTIVE A: DETERMINE THE NATURE AND EVOLUTION OF THE GEOLOGIC PROCESSES THAT HAVE CREATED AND MODIFIED THE MARTIAN CRUST AND SURFACE; INVESTIGATION 2: EVALUATE FLUVIAL, SUBAQUEOUS, PYROCLASTIC, SUBAERIAL, AND OTHER SEDIMENTARY