The Bathymetry and Sediments of Wallowa Lake, Northeast Oregon

Bryce Budlong, J.R. Collier, Calvin Davis, David E. Gilbert, Rob Ledgerwood, and Jay Van Tassell

Science Department- Badgley Hall, Eastern Oregon University,

La Grande, OR 97850-2899

ABSTRACT

Bathymetric profiling of Wallowa Lake shows that the two sub-basins that make up the lake floor have been filled with sediment deposited as the glaciers retreated and by sediment washed in from the Wallowa River across the delta at the south end of the lake. In many areas, the lake floor slopes gently toward the west. There are two west-northwest-trending underwater moraines between the two sub-basins that have reliefs of up to 20 ft (6m) and can be traced out of the water into subtle north-sloping topographic benches on the late Wisconsin glacial moraines on the lake. The southern sub-basin floor is cut by two faults that trend at N60E across the lake floor. The faults form the borders of an ~0.3 mi (0.5 km) wide flat-floored valley (graben) in the deepest part of the south basin.

Sand and gravel were found in depths up to 132 feet (40 m). Below this depth, much of the lake floor is covered with mud that is rich in diatoms, sponge spicules, conifer pollen, and organic material. This mud is frequently laminated (varved?) or mottled, is overlain by sand in some areas near the lake margins, and is underlain in the area between the two underwater moraines by a layer of silt and mud that lacks diatoms.

INTRODUCTION

Wallowa Lake is a world-class example of a glacial lake dammed by moraines. The spectacular moraines around Wallowa Lake rise to 275 m (900 ft) above the lake, and their bases are hidden beneath the lake, which has a maximum depth of 86 m (283 ft; Phillips and others, 1965, p. 65). At the peak of the last ice age, nine large glaciers and their tributaries, plus several other isolated ice fields and glaciers, covered a total of ~337 square miles of the Wallowa Mountains (Fig. 1.) The glacier that occupied the west fork of the Wallowa River was 21 km (13 mi) long and the one that occupied the east fork was 8.9 km (5.5 mi) long. The Wallowa glacier was thickest at its junction with the East Fork (460 m; ~1500 feet) and its terminus reached an elevation of 1280 m (4200 ft; Allen, 1975).

The Wallowa River debouched from the canyon onto a large alluvial fan prior to deposition of the Wallowa Lake moraine (Bentley, 1972.) Then, the moraine was built up in a series of stages as glaciers advanced northward and retreated southward.

The moraine complex on the east side of Wallowa Lake includes more than a dozen individual moraine crests and the west moraine has about half a dozen (Carson, 2001). The complexity of the west moraine complex may be because the ice from the east fork

Figure 1. Extent of glaciers in the Wallowa Mountains at the peak of the last glaciation (Allen, 1975, Orr and others, 1992).

cascaded down onto the ice in the west fork, distributing the material over the valley rather than concentrating it. This caused an increased accumulation of debris on the east side of the glacier which would have been deposited on the eastern margin of the ice as melting occurred, causing the toe of the glacier to be deflected progressively toward the west with each advance of the ice and leaving the more complex moraine now visible on the east side of Wallowa Lake (Stovall, 1929, p. 70; Bentley, 1972).

The number of glacial advances that built the Wallowa Lake moraine complex remains controversial. The compound nature of the moraines bordering Wallowa Lake was first recognized by Stovall (1929), who recognized 8 lateral moraines on the east side of Wallowa Lake and divided these into two groups: 1) an older group of moraines that lack boulders of granodiorite at their surfaces and have subdued morainal topography, and 2) younger moraines which are strewn with granodiorite boulders and have more pronounced morainal topography. Only two episodes of glaciation were recognized by Stovall (1929) and Lowell (1939), while Burke (1980) concluded that there are three ages of tills present in the area and Crandell (1967), who carefully studied the boulders and soils of the Wallowa Lake moraines, concluded that there were 4 episodes of glaciation recorded by the moraines at Wallowa Lake (Fig. 2). The youngest unit recognized by Crandell (1967), Drift Unit W, includes two terminal moraines at the north end of Wallowa Lake, lateral moraines bounding both sides of the lake, an inconspicuous north-sloping lateral moraine that descends the embankment of the east moraine about a mile from the south end of the lake, and a small recessional moraine about a mile further upriver from the lake.

Figure 2. Crandell's (1967) map of the Wallowa Lake moraines plus the bathymetry of Wallowa Lake from the Atlas of Oregon Lakes (Johnson and others, 1985). The moraines, from oldest to youngest, are: C, J, T, and W. Dashed lines indicate moraine crests.

The ages of the Wallowa Lake moraines, based on correlation with other glacial sequences in the Rocky Mountains, Sierra Nevada, and Western Washington areas, have been estimated at ~25,000 to 10,000 years ago and ~ 300,000 to 150,000 years ago (Crandell, 1967) and between ~600,000 to 300,000 years ago (Sibrava and others, 1986). One exciting new development is the dating of boulders on the two younger drift units using high-resolution cosmogenic ¹⁰Be chronology by Licciardi and others (2004). This dating suggests that the second oldest advance is ~ 21,000 years, instead of ~300,000 years old as Crandell (1967) thought, and that the youngest advance is ~17,000 years old. The dates revealed that some of the Wallowa Lake moraines are composite features formed during both glacial episodes. This new evidence suggests that the two older glacial advances may be much younger (300,000 to 150,000 years old?) than earlier thought. This would make the glacial chronology recorded at Wallowa Lake similar to that documented in neighboring mountains such as the Elkhorns. We won't know for sure until age dating of the older moraines becomes available.

After the last glacial advance ~17,000 years ago, the Wallowa glacier retreated uphill to the area of the Glacier Lake cirque, where there is evidence of a series of younger glacial advances that did not reach the Wallowa Lake area. The oldest of these advances is the Glacier Lake advance, which has been dated at ~11,000-10,000 years before present by Licciardi and others (2004) using high resolution cosmogenic ¹⁰Be analysis techniques. Later glacial advances that were confined to the cirques in the Wallowa Mountains have been dated at ~2,600-4,500 years before present (Williams, 1974), between ~175-875 A.D., and between ~1600-1875 A.D. (Kiver, 1974; also see Burke, 1978, and Burke and Birkeland, 1983) based on the diameters of the lichens growing on the boulders in the moraines. Glaciers were reported in the Wallowa Mountains by Smith (1918), Stovall (1929), and Stadter (1931). W.D. Smith and others (1941, p. 36) reported that in 1940 "the last small glacier existing in the Wallowa Mountains is located in a depression on the ridge at the head of Glacier Lake cirque..." The Atlas of Mountain Glaciers (1958) also shows a glacier in this position. By 1972, as Bentley (1974, p. 68-69) reported, "one would be hard-pressed to identify a "glacier" in the Wallowa Mountains."

The Lostine Valley to the west of Wallowa Lake was occupied by a glacier that was 35 km (22 mi) long, the largest in the Wallowa Mountains (Fig. 1). It originated near Eagle Cap (elevation 2925 m; 9,595 ft) in the center of the Wallowas and descended to an elevation of 1030 m (3,380 ft; Allen, 1975). The road along the Lostine Valley crosses a series of end moraines deposited as the glacier retreated up the valley as the glacier retreated. Is it possible that as the Wallowa Glacier retreated similar moraines were left behind on the floor of the Wallowa Lake and were then submerged as the lake level rose?

BATHYMETRY OF WALLOWA LAKE

One piece of evidence supporting the hypothesis that there are submerged glacial moraines on the floor of Wallowa Lake is the inconspicuous north-sloping lateral moraine that descends the east moraine embankment about a mile from the south end of the lake that was mapped by Crandell (1967). The bathymetric chart of the lake prepared by researchers from Portland State University (Johnson and others, 1985) shows that the lake is divided into two sub-basins separated by a ridge which could possibly be a moraine, but the chart is not detailed enough to answer this question for sure (Fig. 2).

Our survey of the floor of Wallowa Lake was made with an Odom Hydrotrac precision depth sounder, which is accurate to ~1 foot when properly calibrated, with a paper chart recorder and built-in Starlink GPS receiver (Fig. 3). A hand-held Garmin GPS receiver was used when the Starlink GPS did not work properly. We ran a total of 24 depth-sounding profiles up and down and across Wallowa Lake in July and August 2002 and collected additional profiles during the summers of 2003 and 2004 (Fig. 4,5).

Figure 3. Setting up the depth sounder and GPS antenna on board the research vessel (top) and close-up view of our Odom Hydrotrac depth sounder during profiling (bottom). Photos by David Gilbert.

Figure 4. Example of a depth sounding record from Wallowa Lake. The chart recorder automatically changes scale when the depth reaches the bottom of the paper. A fish is visible at a depth of 16.5 feet on the left hand side of the profile. The wavy pattern on the right hand side of the profile is due to waves rocking the boat at the surface.

Figure 5. Chart showing the tracks of our bathymetric survey lines.

The results of our bathymetric profiling of Wallowa Lake, digitized and contoured using the geographic information systems (GIS) programs MapInfo and Vertical Mapper, confirm that the lake floor is divided into two sub-basins (Fig. 6). The east-west profiles across Wallowa Lake clearly show the steep sides of the glacial moraines and reveal that the lake floor is not a simple glacial u-shaped valley (Fig. 7). It has been filled with sediment as the glaciers retreated and by sediment washed in from the Wallowa River across the delta at the south end of the lake. In the southern part of the northern sub-basin, the east-west profiles show low-relief ridges and sharp peaks on the bottom of the lake. In many areas, the lake floor slopes gently toward the west.

Figure 6. Bathymetric chart of the floor of Wallowa Lake.

Figure 7. Representative west to east direction bathymetric profiles across the north (top), central, and south (bottom) parts of Wallowa Lake. Profile locations are shown in Figure 5.

The north-south profiles along the length of the lake reveal two west-northwest-trending underwater ridges between the two sub-basins discovered by Portland State researchers (Fig. 8). Each ridge has a relief of up to 6 m (20 ft) and is a compound feature covered with large boulders (glacial erratics). The ridges show up clearly on the bathymetric chart on the east side of the lake but are less defined on the west side of the lake, where they appear to have been eroded away, possibly by glacial meltwater streams flowing along the west side of the lake basin. Each ridge can be traced out of the water into subtle north-sloping topographic benches on the youngest glacial moraines on the lake margin. One of these topographic benches is the inconspicuous lateral moraine described by Crandell (1967). This evidence suggests that the two underwater ridges are glacial moraines deposited sometime after the ~17,000 year old moraines at Wallowa Lake and before the ~10,000 year old Glacier Lake moraines. They may have formed during a glacial advance or could be recessional moraines formed as the ice retreated.

Fig. 8. Bathymetric profile in a south to north direction across the floor of Wallowa Lake.

One surprise revealed by the depth sounding of Wallowa Lake was the discovery of two faults that trend N60E at an acute angle to the main trend of the Wallowa Fault along the base of the mountains and the faults mapped by Walker (1979) in the Wallowa Lake area (Fig. 9). The fault zones are commonly marked by shallow depressions on the depth sounding profiles. This suggests that the faults may be active; otherwise they would tend to be filled with sediment. The faults can be traced from profile to profile across the lake floor and form the borders of an ~ 0.5 km wide flat-floored valley (graben) in the deepest part of the south basin. There is no sign of the faults cross-cutting the moraines on the east side of the lake but the topography on the west side of the lake and on the northeast flanks of Chief Joseph Mountain sloping down toward the lake shows irregularities that may be related to the faulting.

Figure 9. The trends of the two faults mapped on the bottom of Wallowa Lake in comparison with the trend of faults mapped along the shores of Wallowa Lake by Walker (1979).

SEDIMENTS

The sediments in Wallowa Lake have been described as "relatively coarse" (Johnson and others, 1985, p. 140), probably because sand, gravel, and boulders (glacial erratics) are exposed at the shallow north and south ends of the lake and along the lake margins when the lake level is low. Larson (1981, p. 104) stated: "The bottom of Wallowa Lake is covered predominantly in barren sands and gravels, in contact with low temperature water (near 4˚C in summer), and is overlain by 60-80 m of water which effectively keeps the bottom in darkness by filtering incident solar radiation. In general, the lake's bottom represents a biologically unfavorable benthic habitat. In the limnetic or open-water zone, seemingly small supplies of inorganic nutrients...generally restrict the abundance of phytoplankton. As primary energy-fixing organisms in the aquatic food-chain, phytoplankton ultimately determine the growth, production, and survival of crustaceans, insects, fish, and other consumer organisms." The results of plankton tows in Wallowa Lake in 1973 show that the algae Dinobryon divergens accounted for 60 to nearly 100 percent of the phytoplankton biomass. Other phytoplankton sampled included Mallomonas producta, Ceratium hirundinella, and the diatoms Aulacoseira (Melosira) italica and Fragillaria crotonensis.

Our studies have confirmed that the sediments in the shallow portions of Wallowa Lake are coarse-grained. Large boulders (glacial erratics) were apparent on our depth sounding profiles in the north end of the lake. Profiling with a side-scan transponder attached to our depth sounder during the summer of 2003 revealed abundant dead trees around the margins of the lake, particularly on the lake's west shore.

Are the sediments in the deeper parts of the lake coarse-grained, too? We attempted to sample the lake floor sediments in August 2002 using a small pipe dredge. The sediment collected during this survey from the lake bottom within the graben consists of silt with abundant wood fragments, which may have been deposited by the July 18, 2002 flash flood that diverted the Wallowa River and filled the lake with debris (Budlong and others, 2003). We managed to recover some sand samples from the lake margins with the pipe dredge, but encountered problems with the samples washing out when we attempted to sample the deeper portions of the lake floor.

We returned to the lake in the summer of 2004 with a Wildco Ballchek gravity corer, which has a ball at the top that seals the corer and creates a suction when the corer is lifted from the bottom, creating a suction that helps keep the core in the core barrel. Using a hand-powered Wildco winch, we lowered the corer and let it freefall to the lake bottom. Then the corer was winched back to the boat and the sample was extruded using a pipe with a cork on one end, which acted as a piston. Samples that were cohesive enough to maintain their shape were split longitudinally with a knife (Fig. 10). All samples except the final core, which was preserved intact in a plastic core liner, were described on board the boat and then returned to the laboratory for detailed analysis. Smear slides were made from samples taken at 5 cm intervals from each core and examined at 400x power using a Leitz Laborlux Pol petrographic microscope.

Figure 10. Lowering the Wildco Ballchek gravity corer into Wallowa Lake (top) and an example of the one of the cores (bottom). The top of the split core (bottom photo) is to the right. Note the distinctive laminated (varved?) gray mud on the bottom of the core and the brown silty mud overlying the gray mud. A small amount of unconsolidated sand is visible on the top of the core on the right hand side of the photo). The brown color changed to gray when the core dried. Photos by David Gilbert.

The analysis of the cores from Wallowa Lake (Fig. 11) confirmed that the sediments in the deeper portions of the lake are much finer-grained than previously believed (Fig. 12). Sand and gravel were found in depths up to 132 feet. Below this depth, much of the lake floor is covered with mud that is rich in diatoms, sponge spicules, conifer pollen, and organic material. These muds are frequently laminated (varved) or mottled. The most common diatoms present in the lake floor muds are Aulacoseira (Melosira). Other common diatoms include Asterionella, Cocconeis(?), Cyclotella, Cymbella, Diploneis, Epithemia, Fragillaria, Gyrosigma, Navicula, Nitzchia, Stephanodiscus, Synedra, and Tabellaria. This suite of diatoms indicates deep-water conditions and is similar to that found in the modern Columbia and Snake Rivers. Conifer pollen in the cores included Tsuga (hemlock), Picea (spruce), and Pinus (pine). The freshwater sponge Ephydatia fluviatilis was the most common sponge spicule in the lake sediments.

Figure 11. Location of the cores collected from the floor of Wallowa Lake and the type of sediment present at the top of the core. S: Sand. M: Mud. NR: No recovery.

Figure 12. Cross-sections through the northern sub-basin of Wallowa Lake illustrating the types of sediments present in the cores.

The coring revealed that sand overlies the diatom-rich muds in the northern sub-basin of Wallowa Lake north of the northern underwater moraine (Fig. 12). The stratigraphy in the area between the two underwater moraines in the northern sub-basin is more complex. On the western side of this area, the sand on the lake floor is underlain by diatom-rich mud that is underlain by a layer of fine-grained silt and mud that lacks diatoms. On the eastern side of the area between the two underwater moraines, the layer that lacks diatoms directly underlies sand without the intervening diatom-rich mud layer. This sequence of 1) fine-grained mud and silt overlain by 2) laminated (varved) and mottled diatom-rich mud, and 3) sand and coarser sediments suggests that conditions of deposition have changed through time.

Lake sedimentation in regions where the lake and rivers flowing into the lake freeze during the winter is often characterized by a seasonal, rhythmically laminated (varved) depositional regime in which the input of sediment into the lake is cut off and dark, clay-rich layers are deposited from suspension after the winter overturn when clay that has been caught in the thermocline due to the seasonal stratification of the lake is released and settles to the bottom. During the spring, summer, and autumn, coarse sediment is washed into the lake basin by streams and blooms of phytoplankton, including diatoms, result in organic-rich sediments being deposited on the lake floor (Eyles and Eyles, 1992; Talbot and Allen, 1996). The diatom-rich mud present in the deeper parts of the lake appears to have been deposited by this mechanism.

The origin of the laminated fine-grained silt and mud that lacks diatoms is less clear. It is possible that these sediments were deposited at a time when there were not enough nutrients or too much fine-grained sediment in the lake water for diatom blooms to occur. Another possibility is that these sediments were deposited when the level of Wallowa Lake was low enough for the southern underwater moraine to act as a barrier, trapping coarse sediment in the southern part of the lake and only allowing fine-grained sediment to be deposited by overflows and interflows passing over the moraine into the northern sub-basin. A third possibility is that these sediments are wind-blown (loess) deposits (Fig. 13).

Sand has built out over the top of the older layers. It is possible that some of the sand was washed in by the Wallowa River and carried along the lake margins by wave-generated longshore currents, but it seems likely that most of the sand has come from the winnowing of sand out of the glacial till along the margins of the lake during storms. The deposition of sand on top of the older layers may be related to the drop in lake levels indicated by the lake sediments present above the present day lake level that were described by Carson (2001).

Figure 13. Hypotheses for the origin of the silt and mud unit that lacks diatoms.

Figure 14. Generalized south-north geologic cross-section across the Wallowa Lake area.

GEOLOGIC HISTORY OF WALLOWA LAKE: A SUMMARY

As the Wallowa Mountains continued to uplift along the Wallowa Fault and climate began to cool at the dawn of the Ice Ages around 3 million years ago, erosion formed large alluvial fans at the base of the mountains. Glaciers formed as snow accumulated faster than it melted and tongues of ice advanced down the mountain valleys and out across the older alluvial fan sediments. The ice advanced and retreated several times, leaving behind the complex series of glacial moraines that frame present-day Wallowa Lake (Fig. 14). As the glacial ice retreated the final time, it grew thinner, leaving behind moraines at progressively lower elevations. These moraines may have been deposited during brief pauses in the retreat of the ice. Or, they may represent minor re-advances of the ice that did not match the extent and height of those that preceded them. As the ice retreated up the valleys into the High Wallowas and finally disappeared, sediment carried by glacial meltwater streams was deposited in the south end of the lake, forming delta deposits that built out into the lake and upward as the lake level rose. Fine-grained silt and mud, perhaps deposited during low lake levels or by wind action, accumulated between the two submerged moraines and were covered by mud rich in diatoms and conifer pollen. This mud fills a fault-bounded graben in the deepest part of the lake, evidence that tectonic activity has continued in the area in recent times. Today, the Wallowa River is small compared to what it must have been like as glaciers retreated from the area, but deposition on the delta and in the lake continues, especially in the springtime when the river is swollen by snowmelt and when summer thunderstorms rapidly drop large amounts of precipitation into the high mountains that tower above the lake.

CONCLUSIONS

Our bathymetric profiling of Wallowa Lake confirms that the lake floor is divided into two sub-basins as previously suggested by Oregon State University researchers. East-west profiles across Wallowa Lake show that the lake floor has been filled with sediment as the glaciers retreated and by sediment washed in from the Wallowa River across the delta at the south end of the lake. In many areas, the lake floor slopes gently toward the west. North-south profiles along the length of the lake reveal two west-northwest-trending underwater moraines between the two sub-basins. These moraines are up to 6m (20 ft) high and are covered with large boulders (glacial erratics). The moraines can be traced out of the water into subtle north-sloping topographic benches on the late Wisconsin (Drift unit W) glacial moraines on the lake. Our bathymetric profiling also revealed two faults that trend N60E at an acute angle to the main trend of the Wallowa Fault along the base of the mountains and the faults that have been previously mapped in the Wallowa Lake area. The faults form the borders of an ~ 0.5 km (0.3 mi) wide flat-floored valley (graben) in the deepest part of the south basin.

The sediments in the bottom of Wallowa Lake were previously identified as

sands and gravels. Our studies have confirmed that the sediments in the shallow portions of Wallowa Lake are coarse-grained. Sand and gravel were found in depths up to 40 m (132 feet). Below this depth, much of the lake floor is covered with mud that is rich in diatoms, sponge spicules, conifer pollen, and organic material. This mud is frequently laminated (varved?) or mottled and is overlain by sand in some areas. The diatom-rich mud is underlain in the area between the two underwater moraines by a layer of silt and mud that lacks diatoms. On the eastern side of the area between the two underwater moraines, the layer that lacks diatoms directly underlies sand without the intervening diatom-rich mud layer.

The layer of fine-grained mud and silt that lacks diatoms at the bottom of our cores may record the deposition of silt and finer particles washed over the southern submerged moraine as the lake filled. Or, this layer could have been deposited by wind action. This was followed by deposition of diatom-rich sediments as the lake deepened and winnowing of sand by wave action from the glacial tills that form the lake margins. Another possibility is that the deposition of sand on top of the older layers is related to the drop in lake levels indicated by the lake sediments present above the level of the present day lake.

Unfortunately, we do not know the ages of these sediments. Dating the organic material in the Wallowa Lake cores using radiocarbon and other techniques would help us to better understand the events during the deposition of the sediments in Wallowa Lake. And, since the maximum depth penetrated by our coring was only 61 cm (2 ft), it is likely that there are other sediment layers beneath the ones that we sampled that may help fill in more of the geologic history of Wallowa Lake. Longer cores obtained with a larger corer or by vibracore drilling, coupled with seismic profiling of the lake floor, would add a great deal to our knowledge of the geologic history of Wallowa Lake and the surrounding area.

ACKNOWLEDGMENTS

We would like to thank Carolyn Gilbert for her wonderful hospitality as we were conducting our field research on Wallowa Lake. We gratefully acknowledge the Eastern Oregon University Tech Fee Committee for making it possible for us to purchase the depth sounder, side-scan sonar transponder, winch, and corer that made this research possible. Odom Hydrographic, Inc., repaired our depth sounder several times over the course of this investigation.

REFERENCES CITED

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