The Value of Reservoir-Bottom Field Trips for ... Teachers of sediment-related geology courses...
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ABSTRACT Teachers of sediment-related geology courses should consider taking their classes on field trips to exposed reservoir bottoms. A wonderful variety of fluvial, deltaic, and lacustrine features and processes can be observed there. A visit to a modern sedimentary environment such as a reservoir bottom gives students important personal experience that they can draw on when learning to interpret sedimentary rocks using the principle of uniformitarianism. It can also inspire investigation of a range of related geological and environmental issues. INTRODUCTION When planning field trips, most geology professors teaching courses such as Historical Geology, Sedimentology, and Stratigraphy look for field sites with good rock outcrops, roadcuts, and other exposures where students can learn how to interpret the sedimentary rock record by applying the principle of uniformitarianism. To experienced sedimentary geologists, interpretation of depositional environment from characteristics such as grain size and sedimentary structures is fairly intuitive, but it can be difficult for students who have little or no personal knowledge of sedimentary processes. There are laboratory apparati that can help remedy this situation, such as stream tables and flumes, that allow students to experiment with erosional and depositional processes (Lillquist and Kinner, 2002; Field, 2003). Field trips to visit rivers, lakes, estuaries, dunes, glaciers, and other geomorphically active areas provide an "eyewitness" view of natural features and processes. This kind of experience helps strengthen the connection students feel to the material they learn about in the classroom (Heins and Walker, 1998; O'Connell et al., 2004). Uniformitarianism then becomes a tool in which students can apply their personal experience to the interpretation of sedimentary rocks. In my experience as an environmental geologist who began studying lakes and reservoirs for my doctoral research several years ago, I have learned that reservoir bottoms are a superb natural sedimentology field area. When reservoirs are drawn down so that the lake bottom is exposed, conditions are ideal for observation of sedimentary processes, structures, and depositional environments. Depending on the conditions and topography of a particular reservoir, observation of fluvial, deltaic, lacustrine, littoral, and aeolian features is possible. Recent fluctuations in lake level due to flood events and river regulation leave behind abundant evidence of the processes at work in moving sediment around. The result is an excellent field trip site for courses dealing with sedimentary rocks and also for those dealing with geomorphic processes (Twidale, 1999). Not every college and university has a suitable reservoir close enough for a field trip, but for those that do, the teaching potential of the trip makes it well worth the effort. My purpose in writing this article is (a) to encourage teachers of sediment-related courses to consider including a field trip to an exposed reservoir bottom and (b) to provide some ideas for the kinds of activities that can be performed on the trip. RESERVOIR-BOTTOM FEATURES AND ACTIVITIES When the water level of a reservoir is dropped significantly, the submerged stream valley, now coated with an accumulation of lake sediment, is once again exposed to air and sunlight. Depending on the age of the reservoir, local sediment supply, topography of the valley, and reservoir sedimentation dynamics, sediment deposits may be thick or thin, and this often varies quite a bit from place to place on the reservoir bottom. As the water level shallows during drawdown, wind-driven waves and currents rework reservoir-bottom sediments, frequently forming structures such as ripple marks. As clay-rich sediment becomes exposed to air and pore waters drain, mudcracks develop and oxidation takes place in the upper layers. If conditions on the reservoir bottom have been dry for some time, windblown sand ripples and other aeolian features may also form. The mainstem and tributary streams that supply water to the reservoir flow freely across the exposed valley bottom once again, eroding their banks and flushing downstream the fine-grained lake sediment that accumulated in their channels. These streams also experience flooding, and overbank events produce particularly striking effects on the exposed reservoir-bottom sediments. Flood events also usually result in a rise and fall of lake level (although the exact response is affected by reservoir management practices). Submersion and re-exposure of the valley bottom by the reservoir pool produces additional erosional and depositional features, including temporary deltas located where sediment-laden floodwaters reached the still pool of the elevated reservoir. From a teaching perspective, the end result of all of these processes is a field area rich in learning opportunities for students. There is a great deal to discover, and most features of interest to a sedimentary geology class are fairly easy to access, although some walking will be required. Table 1 provides a list of sedimentary structures and features that can be observed on exposed reservoir bottoms, and selected photographs of these features from three different reservoirs are shown in Figure 1. In many cases, I have found two or more sedimentary structures superimposed upon one another, providing an interesting opportunity for students to apply the principles of stratigraphy to sort out which formed first, second, and so on. Examples include invertebrate trails on top of bird trackways (Figure 1A), ripple marks that were subsequently mudcracked, and footprints that were later rippled. Also 508 Journal of Geoscience Education, v. 53, n. 5, November, 2005, p. 508-512 The Value of Reservoir-Bottom Field Trips for Undergraduate Geology Courses Rebecca K. R. Ambers Department of Environmental Studies, Sweet Briar College, Sweet Briar, VA 24595, [email protected]
Transcript of The Value of Reservoir-Bottom Field Trips for ... Teachers of sediment-related geology courses...
ABSTRACT
Teachers of sediment-related geology courses shouldconsider taking their classes on field trips to exposedreservoir bottoms. A wonderful variety of fluvial,deltaic, and lacustrine features and processes can beobserved there. A visit to a modern sedimentaryenvironment such as a reservoir bottom gives studentsimportant personal experience that they can draw onwhen learning to interpret sedimentary rocks using theprinciple of uniformitarianism. It can also inspireinvestigation of a range of related geological andenvironmental issues.
INTRODUCTION
When planning field trips, most geology professorsteaching courses such as Historical Geology,Sedimentology, and Stratigraphy look for field sites withgood rock outcrops, roadcuts, and other exposureswhere students can learn how to interpret thesedimentary rock record by applying the principle ofuniformitarianism. To experienced sedimentarygeologists, interpretation of depositional environmentfrom characteristics such as grain size and sedimentarystructures is fairly intuitive, but it can be difficult forstudents who have little or no personal knowledge ofsedimentary processes. There are laboratory apparatithat can help remedy this situation, such as stream tablesand flumes, that allow students to experiment witherosional and depositional processes (Lillquist andKinner, 2002; Field, 2003). Field trips to visit rivers, lakes,estuaries, dunes, glaciers, and other geomorphicallyactive areas provide an "eyewitness" view of naturalfeatures and processes. This kind of experience helpsstrengthen the connection students feel to the materialthey learn about in the classroom (Heins and Walker,1998; O'Connell et al., 2004). Uniformitarianism thenbecomes a tool in which students can apply theirpersonal experience to the interpretation of sedimentaryrocks.
In my experience as an environmental geologist whobegan studying lakes and reservoirs for my doctoralresearch several years ago, I have learned that reservoirbottoms are a superb natural sedimentology field area.When reservoirs are drawn down so that the lake bottomis exposed, conditions are ideal for observation ofsedimentary processes, structures, and depositionalenvironments. Depending on the conditions andtopography of a particular reservoir, observation offluvial, deltaic, lacustrine, littoral, and aeolian features ispossible. Recent fluctuations in lake level due to floodevents and river regulation leave behind abundantevidence of the processes at work in moving sedimentaround. The result is an excellent field trip site forcourses dealing with sedimentary rocks and also forthose dealing with geomorphic processes (Twidale,1999).
Not every college and university has a suitablereservoir close enough for a field trip, but for those thatdo, the teaching potential of the trip makes it well worththe effort. My purpose in writing this article is (a) toencourage teachers of sediment-related courses toconsider including a field trip to an exposed reservoirbottom and (b) to provide some ideas for the kinds ofactivities that can be performed on the trip.
RESERVOIR-BOTTOM FEATURES ANDACTIVITIES
When the water level of a reservoir is droppedsignificantly, the submerged stream valley, now coatedwith an accumulation of lake sediment, is once againexposed to air and sunlight. Depending on the age of thereservoir, local sediment supply, topography of thevalley, and reservoir sedimentation dynamics, sedimentdeposits may be thick or thin, and this often varies quite abit from place to place on the reservoir bottom. As thewater level shallows during drawdown, wind-drivenwaves and currents rework reservoir-bottom sediments,frequently forming structures such as ripple marks. Asclay-rich sediment becomes exposed to air and porewaters drain, mudcracks develop and oxidation takesplace in the upper layers. If conditions on the reservoirbottom have been dry for some time, windblown sandripples and other aeolian features may also form.
The mainstem and tributary streams that supplywater to the reservoir flow freely across the exposedvalley bottom once again, eroding their banks andflushing downstream the fine-grained lake sediment thataccumulated in their channels. These streams alsoexperience flooding, and overbank events produceparticularly striking effects on the exposedreservoir-bottom sediments. Flood events also usuallyresult in a rise and fall of lake level (although the exactresponse is affected by reservoir management practices).Submersion and re-exposure of the valley bottom by thereservoir pool produces additional erosional anddepositional features, including temporary deltaslocated where sediment-laden floodwaters reached thestill pool of the elevated reservoir.
From a teaching perspective, the end result of all ofthese processes is a field area rich in learningopportunities for students. There is a great deal todiscover, and most features of interest to a sedimentarygeology class are fairly easy to access, although somewalking will be required. Table 1 provides a list ofsedimentary structures and features that can be observedon exposed reservoir bottoms, and selected photographsof these features from three different reservoirs areshown in Figure 1. In many cases, I have found two ormore sedimentary structures superimposed upon oneanother, providing an interesting opportunity forstudents to apply the principles of stratigraphy to sortout which formed first, second, and so on. Examplesinclude invertebrate trails on top of bird trackways(Figure 1A), ripple marks that were subsequentlymudcracked, and footprints that were later rippled. Also
508 Journal of Geoscience Education, v. 53, n. 5, November, 2005, p. 508-512
The Value of Reservoir-Bottom Field Trips for UndergraduateGeology CoursesRebecca K. R. Ambers Department of Environmental Studies, Sweet Briar College, Sweet Briar, VA
24595, [email protected]
of value for students are opportunities to viewsedimentary structures both from the top and incross-section along reservoir-bottom stream banks(Figure 1D).
Beyond sedimentary geology, a trip to a reservoirbottom can spark exploration of a wide range of othergeological and environmental topics. The redoxboundary in reservoir sediment provides a graphicillustration of biogeochemical processes, as do springsand seeps which frequently exhibit orange microbialdeposits and a sulfurous smell (Carlson, 1999). If thereare upstream sources of chemical pollution, students canexplore the wider impacts of contamination on aquaticand terrestrial ecosystems (Heins and Walker, 1998).Water quality and resource management, watershed soilerosion, plant community succession in disturbed areas,and the impact of dams and dam removal are otherissues that can be related to observations made on thetrip (Devine, 1995; Lee, 1998; Panno et al., 1998;Dunnivant et al., 1999; Graf, 1999; Bednarek, 2001; Grant,2001; Salvage et al., 2004).
While visiting an exposed reservoir bottom, studentscan collect a wide variety of data as part of field tripexercises, and sediment and water samples can bebrought back for use in later laboratory exercises.Integration of field and lab exercises within the
framework of a course can be an effective way to engagestudents in the learning process (Trop et al., 2000). Table2 lists some ideas for activities that could be used invarious courses.
TIPS FOR FIELD TRIP PLANNING
Finding an appropriate reservoir for a field trip mayrequire some searching because not all reservoirsexperience significant enough drawdown to be useful.The first step is to check on the ownership andmanagement of reservoirs within a reasonable distancefrom the college or university. Often, there is aconsiderable amount of online information about largeprojects, such as flood-control and hydroelectric dams,and associated recreational facilities operated by the U.S.Army Corps of Engineers (http://www.usace.army.mil/where.html) and the U.S. Bureau of Reclamation(http://www.usbr.gov/dataweb/). Some Army Corpsdistrict web pages even provide graphs showing theannual management schedule and live-time data onreservoir water levels, which makes it easy to plan asuitable date for a trip. Although vehicle access to somedams has been restricted since the events of September11, 2001, going out on the reservoir bottom is generallynot a problem at public reservoirs where parking areas,lakeside campgrounds, and boat launches are provided.Dam tours may even be available upon request.Reservoirs operated by municipalities and privately
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Laminated sediment; individual flood events often produce silt-clay coupletsChanges in grain size with depositional environmentSmall- to medium-sized deltas where streams enter thereservoir poolCross-beddingGraded bedding; reverse grading is difficult to findbecause of the nature of the sedimentary environmentLocalized concentrations of heavy minerals or organicmaterialRipple marks of different sizes and types, with occasionaldunes and anti-dunes depending on water velocity;sometimes windblown ripples are also presentRip-up clasts, including ripped up mudcrack polygonsMudcracks of different sizes and depthsRaindrop imprintsAnimal, human, and vehicular trackways and footprints,including those of birds, mammals, and invertebrates;analogous to trace fossilsArcuate "swish marks" from grasses moved back and forthby windScour around flow obstructions; often combined withsediment deposits or other features on the leeward side ofthe obstructionDrapage of lake sediment onto preexisting topographyStreambank slumps and other collapse featuresThe "bathtub ring" and/or wavecut terrace(s) that markhigh water levels for the reservoirPre-reservoir soil profiles buried beneath layers of lakesediment; these resemble paleosols in many waysHistorical archaeological features such as railroad beds,road beds, bridge abutments, garbage dumps, buildingfoundations, etc., and artifacts such as beverage containersand fishing gear embedded in reservoir sediment
Table 1. Sedimentary structures and features thatcan be found on reservoir bottoms.
Hold a scavenger hunt for sedimentary structures. Giveeach student team a list of structures to look for and adigital camera to record their finds. Give students a timelimit and have them meet back at a specified location toshare observations and determine the winning team.Use a gouge auger or other type of soft-sediment corer totake intact lake sediment cores, or use a shovel or puttyknife to clean off a suitable cross-sectional exposure ofsediment. Have students examine the sediment in detailand measure and describe its stratigraphy. Compare coresor exposures from different sedimentary environments tolook for differences. Have students speculate on the causesof the features they observe. See if any beds or features canbe correlated from place to place.Have student groups use a coring device and a measuringtape or GPS to map sediment thickness on a grid over aspecified area. Back in the lab, students can create anisopach map of their data and calculate the total volume ofsediment contained in that area. If the total area and age ofthe reservoir are known, these data could be extrapolatedto estimate a minimum sediment yield or denudation ratefor the reservoir's watershed. Have students collect sediment samples and take themback to the lab for later analysis. Depending on how thetrip fits into a particular course, analysis could includemeasuring and plotting up grain size distribution,determining mineralogy and lithology of source rocksusing a microscope and/or thin sections, and determiningthe chemical composition of the sediment.Have students collect samples of water inflow to thereservoir from the primary stream(s), the lake water, andreservoir outflow to test the turbidity and suspendedsediment concentration. Additional water quality analysescould be done in environmentally oriented courses to testfor nutrients and pollutants.
Table 2. Ideas for field trip and follow-up laboratoryexercises.
owned lakes are less likely to have information onlineand may require special permission to access.
Reservoir water levels fluctuate for a variety ofreasons. Some reservoirs, such as the Army Corps ofEngineers' projects in the Willamette Valley of westernOregon, have a planned, annual cycle of drawdown andfill-up to facilitate flood control during the rainy season(Figure 2). Other reservoirs, such as those in manynorthern U.S. states, are partially drained in late winterto make room for storage of spring snowmelt runoff.Some flood-control projects are operated as "dry dams"which normally maintain only a very small pool,allowing river flow to pass through the dam exceptduring flood events when the whole reservoir may fill upthen be slowly drained out to prevent overbank floodingdownstream. Reservoirs of all sorts experiencedrawdown during times of drought or when the damrequires repair or servicing. Whatever the reason fordrawdown, flood events can bring up reservoir waterlevels fairly quickly, so it is best to check conditions justbefore the field trip.
As with any field trip, it is important to bring theappropriate gear (Table 3). Everyone who goes out on thelake bottom should wear rubber boots that come up atleast to mid-calf (knee-high is best). Some students canget by with rugged sandals, but they must be prepared toget wet, muddy feet. As for field equipment, measuringtapes and grain size cards are useful for making
measurements of sediment characteristics andstructures. A simple shovel is a convenient tool to use tosee features at depth. A gouge auger is useful for takingintact cores for immediate observation, or rigid,thin-walled plastic tubes can be pounded into thesediment and then excavated and the core extruded. Tobring sediment back to the lab for further analysis, plasticreclosable bags are sufficient for disturbed samples, orcores can be placed on a piece of gutter pipe or split PVCpipe and then secured in a waterproof box. Watersamples should be taken in plastic bottles appropriate forthe type(s) of analysis planned.
The primary safety concern on reservoir bottoms ispeople getting stuck in the mud. This is usually morefrightening than it is dangerous, but it is best not to takechances. As long as the sediment is not water-saturated,walking is usually not difficult, as silty reservoirsediment is firm when it has had a chance to drain for awhile. If there are mudcracks, the footing is usually fine.Be cautious, however, and walk slowly on untestedground. Avoid areas where there is a film of waterstanding on top of the sediment or where springs orseeps are evident. Do not send students out alone, andhave them walk one behind the other rather thanside-by-side. That way, if the first person gets stuck, theothers can come to his/her aid. If a person starts sinkinginto the mud, they should back up immediately, asstanding still will only result in sinking deeper. If a
510 Journal of Geoscience Education, v. 53, n. 5, November, 2005, p. 508-512
Figure 1. Examples of sedimentary structures and features that can be observed on exposed reservoirbottoms. (top left) Bird and arthropod trackways. (top right) Mudcracks partially filled in with sediment.(bottom left) Ripple marks and dunes. (bottom right) Asymmetric ripple marks in cross-section along ariverbank with pencil for scale.
mucky area must be crossed, walk as quickly as possibleto avoid getting stuck.
In some locations where a stream cuts across thereservoir bottom, streambank collapse can be an issue aswell. Reservoir sediment is easily eroded, and a streamcan do a tremendous amount of work on recentexposures of such sediment. As always, common senseshould be applied in this situation. If collapse featuresare evident or if the stream is undercutting its banks,keep back from the edge.
A successful reservoir-bottom trip depends on anumber of uncontrollable factors including the weatherand reservoir management decisions, so the instructorneeds a contingency plan in case the reservoir becomesinaccessible. During a pre-trip scouting visit, it isadvisable to take photos of interesting reservoir-bottomfeatures and bring back some sediment and/or watersamples that students could work with if the trip cannotbe held. An online exercise could also be developedwhere students research the reservoir's history andmanagement if this is relevant to the course. In the case ofrainy weather, it may still be worth making the tripbecause observing the river and reservoir in flood from asafe distance can be very interesting.
CONCLUSION
Most geologists teaching historical and sedimentarygeology courses take their students on field trips to seerocks units that are thousands to billions of years old.Giving students a chance to observe modernsedimentary processes is a great way to help them learnhow to interpret these ancient deposits. It provides themwith personal experience to draw on when applying the
Ambers - Reservoir-Bottom Field Trips 511
Figure 1. Examples of sedimentary structures and features that can be observed on exposed reservoirbottoms. (top left) Ripple "shadows" in the lee of cobbles protruding from reservoir sediment; unidirectionalflow was from an overbank flood. (top right) Prodelta slope where a tributary stream normally enters thereservoir. (bottom left) Thick deposits of reservoir sediment exposed along the bank of a reservoir's mainstemstream. (Bottom right) A tent complete with poles (to the right of the shovels) buried within reservoirsediment downstream from a creekside campground. Photos A and C were taken at Cottage Grove Lake andphoto E at Dorena Lake, both near Cottage Grove, Oregon; the other photos were taken at WaterburyReservoir near Stowe, Vermont.
Rubber boots for everyoneShovelSediment corerMeasuring tapeGrain size cardPlastic reclosable bags for sediment samplesPlastic bottles for water samplesDigital camera
Table 3. Suggested packing list for the trip.
principle of uniformitarianism. Exposed reservoirbottoms are great places to show studentssedimentologic processes in action and to hone theirobservational skills. It can also be the starting point foradditional laboratory exercises that utilize data andsamples collected on the trip. With some planning, a fieldtrip to an exposed reservoir bottom can thus be anexcellent teaching tool.
ACKNOWLEDGMENTS
I thank Beverley Wemple of the University of Vermontfor hosting me on the two trips I took to visit WaterburyReservoir during 2002. I also thank my husband andfellow geologist, Cliff Ambers, for his enthusiasm andenergetic field assistance during our many reservoir fieldtrips.
REFERENCES
Bednarek, A.T., 2001, Undamming rivers: A review ofthe ecological impacts of dam removal,Environmental Management, v. 27, p. 803-814.
Carlson, C.A., 1999, Field research as a pedagogical toolfor learning hydrogeochemistry and scientific-writing skills, Journal of Geoscience Education, v. 47,p. 151-157.
Devine, R.S., 1995, The trouble with dams, AtlanticMonthly, v. 276, p. 64-74.
Dunnivant, F.M., Brzenk, R., and Moore, A., 1999, Acomprehensive stream study designed for anundergraduate non-majors course in earth science,Journal of Geoscience Education, v. 47, p. 158-165.
Field, J., 2003, A two-week guided inquiry project for anundergraduate geomorphology course, Journal ofGeoscience Education, v. 51, p. 255-261.
Graf, W.L., 1999, Dam nation: A geographic census ofAmerican dams and their large-scale hydrologicimpacts, Water Resources Research, v. 35, p.1305-1311.
Grant, G., 2001, Dam removal: Panacea or Pandora forrivers?, Hydrological Processes, v. 15, p. 1531-1532.
Heins, W.A. and Walker, J.R., 1998, Using a campuswaterway for undergraduate-course exercises andsummer-research experiences, Journal of GeoscienceEducation, v. 46, p. 45-50.
Lee, M., 1998. Hands-on laboratory exercises for anundergraduate hydrogeology course, Journal ofGeoscience Education, v. 46, p. 433-438.
Lillquist K.D. and Kinner, P.W., 2002, Stream tables andwatershed geomorphology education, Journal ofGeoscience Education, v. 50, p. 583-593.
O'Connell, S., Ortiz, J., and Morrison, J., 2004, Connectingurban students with their rivers generates interestand skills in the geosciences, Journal of GeoscienceEducation, v. 52, p. 462-471.
Panno, S.V., Hackley, K.C., and Nuzzo, V.A., 1998,Teaching multidisciplinary environmental science ina wetland setting, Journal of Geoscience Education,v. 46, p. 157-163.
Salvage, K., Graney, J., and Barker, J., 2004,Watershed-based integration of hydrology,geochemistry, and geophysics in an environmentalgeology curriculum, Journal of GeoscienceEducation, v. 52, p. 141-148.
Trop, J.M., Krockover, G.H., and Ridgway, K.D., 2000,Integration of field observations with laboratorymodeling for understanding hydrologic processes in an undergraduate earth-science course, Journal ofGeoscience Education, v. 48, p. 514-521.
Twidale, C.R., 1999, A plea for the best of thepast-Suggestions for teaching about landforms,Journal of Geoscience Education, v. 47, p. 241-248.
512 Journal of Geoscience Education, v. 53, n. 5, November, 2005, p. 508-512
Figure 2. Water year 1990 lake levels for Dorena Lake, a reservoir near Cottage Grove, Oregon. The rule curverepresents the lake levels the U.S. Army Corps of Engineers tries to maintain each year to allow for floodcontrol and water storage. The best time to visit a reservoir like this is during the winter months.
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