THE DUNE RIDGES OF CLATSOP COUNTY
BY
FRANK RECKENDORF, CURT PETERSON, AND DAVID PERCY
Open File Report O-01-07
STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES
JOHN D. BEAULIEU, STATE GEOLOGIST
INTRODUCTION
Three small scale studies of the sand dunes in Clatsop Co. in Oregon have been published (Cooper, 1958; Reckendorf, 1975; Rankin, 1983). All three studies recognized that the Clatsop Co. dunes are a series of parallel ridges with some difference in length and height. Recent work along the Columbia River cell as part of research project "USGS Southwest Washington Beach Erosion" has revisited the Clatsop Plains area of Clatsop Co. and the dunes were remapped using large- scale photography and topographic maps.
METHODS
The Clatsop Plains dunes were remapped at 1:6,000 (1-inch equals' 500 ft.) photography dated 9/9/95, using a stereoscope. These ridge lines were then transferred from the individual nine by nine-inch photographs to a photomosaic at the same scale to determine the dunes linear characteristics over several miles. The ridges drawn were then compared with ridges apparent on 1:1,200 (1 inch equals 100 ft.) topographic maps prepared in 1973. Adjustments were made to the stereoscope determined dune lines to reflect small dune ridges apparent on the very large scale topographic mapping. The photomosaic maps where than taken to the field for correlation of the dune ridges. Field correlation involved walking or driving along or across the dune ridges to establish that the same dune was being mapped. Consistency in the dunes sequence was also established by comparing juxtaposition, dune height, and interdune wetlands and lakes.
Historical photos, such as shown in Figures 1 and 4, were also used to help establish modern dune overlap of older dunes in about the last 70 years.
RESULTS
The Clatsop Co. dunes occur in an area that is locally referred to as the Clatsop Plains.
At the large scale of mapping it was apparent that the dunes could be separated into groups of similar dunes based on height, width and juxtaposition, with occasional convergence of adjacent ridges. That parallel dunes tend to occur in similar groups was apparent to an extent in the Cooper (1958) and Rankin (1983) work. However, the ridges are often less than 60 to 90 meters apart, so unless mapped at a large scale, they appear as the same ridge, or as dunes that have converged.
The group sequence consist of the two youngest dunes labeled 1 and 2; a dune sequence labeled 3a, 3a 3b, 3b 3c, 3d; a dune sequence labeled 4a, 4b, 4c, and 4d; and a dune sequence labeled 5a, 5b, 5c, 5d, and 5e; for a total of 17 dune ridges in Clatsop County. The parallel dune ridges are shown on the map in different colors. Since this study was part of a regional study that extends up through southwest Washington, the Oregon dune ridges were correlated to equivalent dune ridges in Washington using dune form, positions, soils and C-14 dates. However, the progradation history along the southwest Washington shoreline in the last 100 years was much more rapid than on the Oregon shoreline. In addition a continuous double row sand fence associated with extensive European beachgrass plantings did not occur along the southwest Washington shoreline, as had occurred along the northwest Oregon shoreline. As a result, a series of small dune ridges 1a, 1b, and 1c were locally developed in Washington, particularly at the spit ends. Therefore the map legend for the Clatsop Plains reflects the additional la, 1b, and 1c ridge sequence even though the time frame of these small ridges are all included in the ridge labeled 1, in the Clatsop Plains.
Dune height was useful in the field correlation in the Clatsop Plains because the youngest two dunes, the modern foredune and the foredune (2nd dune) that was present prior to the jetties, or about 1900 AD, occur roughly at a 10 meter (33 ft.) dune height. The 10 meter modern foredune labeled 1 occurs along the alignment of a double row sand fence (Figure 2) that was planted with European beachgrass in the mid 1930s (Reckendorf, et al 1985). This dune extends for 21 km. (13 miles) from Seaside to the Columbia River. The foredune grew rapidly as the European beachgrass (Figure 3) could withstand up to three feet of winter burial. The 2nd dune is almost as long as the 1st dune and extends from Seaside for about 11 miles to the north. The 3rd dunes are all roughly 15 to 20 meters (49 to 66 feet) in height, so it is easy to separate the 2nd dune, that is present throughout most of the Clatsop Plains from the 3rd dune sequence, even though they may occur within 60 meters (197 feet) of each other. The height of the 3rd dune sequence is somewhat confused because of the modern dune overlay (Figures 1 and 4) on the older 3rd dune sequence (Reckendorf, 1995). The 3d and 3c dunes are the only dunes in the Clatsop Plains to occur locally as a wide bench top hundreds of meters wide. The majority of the truncated dune (breaches) in the Clatsop Plains occur in the 3rd dune sequence. The dunes in the 4th dune sequence are also roughly 15 to 20 meter (49 to 66 feet) in dune height. They may be very close (within 50 meters, 164 feet) of each other, with adjacent dunes tending to range from a 50 to 90 meters (164 to 296 feet) spacing apart. The dunes in the 5th sequence occur further apart than in the 4th sequence and they are a mixture of both 15 meter, 10 meter, and 5 meter (49, 33, and 16 feet) dune height.
The lakes and wetlands in the Clatsop Plains have the same alignment as the dunes. Therefore if a breach in the dune linear extent was found in mapping, than a continuation along the interdune wetlands and lakes was used to establish the correct dune ridge on the far side of the breach.
Historical photos, such as the oblique aerial photo 1933-1934 (Figure 1) show the eastern extent of modern dune activity and overlap of older dunes. The long double row sand fence that was established in the fall of 1935 is not apparent in the photo, and it is estimated that the photo was taken during a winter storm period one or two years prior to 1935. The Figure 1 photo of the Sunset Lake area, reflects complete overtopping and burial of dune 2. Also shown is the partial burial (high ridge points are still exposed) of dune 3a along with the scour exposure of the dunes paleosol along a road cut. East of dune 3a the modern dune activity has almost completely covered the ridge of dune 3b along the eastern extent of the new sand, and into dune 3c on the north end. The April 1936 photograph looking west (Figure 4), shows burial of dune 3b in the foreground, and a remnant of 3a dune in the center of the photograph in the background. The 1974 oblique photograph looking south in the Clatsop Plains (Figure 5), shows the modern foredune (dune 1) along the right of the photograph, just above the beach line. The 1974 dune activity has completely overtopped the modern foredune (just above the beach) and moved into the back dune area behind dune 1 where there is a small ridge. In the center of the photograph there is a road that jogs from right to left. To the right of where the photographic has jogged to the left there is a row of trees and to the right of these trees a row of houses. This row of houses sites on dune 2, which is the pre-jetty foredune. To the left of where the road jogs left, there is some vegetation and a lake and wetlands and than a high dune. This high dune is dune 3a.
Radio Carbon dates within the Clatsop Plains from Woxell (1998) are shown from north to south in Table 1. Samples shown reflect a variety of dating materials, physical settings, and depth to sampling. Therefore the dates should not be considered as sequential from right (east) to left (west). However, the back date of 4050yrs. BP does reflect that beach progradation started after that time. The 4050 yrs. BP date is for a sample 8.2 meters deep and represents a lag of about 1000 years after progradation started. This reflects a back date of about 5000 yrs. BP. Using the 5000yrs. BP back date the rate of beach accretion out to dune number 2 is 0.5 m./yr. (Peterson et al, 1999). In contrast the rate from dune 2 to dune 1 varies from 2.75 at Gearhart to 5.75 up near the south jetty (Peterson et al, 1999). The modern dune activity buried the historical soil profiles along the 2nd dune and the windward side of the 3a dune (Figure 1), so the buried soil dates of 160 yrs. BP and 180 yrs. BP represent the recent historical soils, and show young dates as expected.
| NAME | MATERIAL SAMPLED | DEPTH (m) |
STRATIG. SETTING |
LOCATION |
RC DATE BEFORE PRS. |
| SECO1 | PALEOSOL | 4.0 | RECENT SND. | ON 3b DUNE | 180+/-60BP |
| RANK6 | PEAT | 2.8 | PEAT | LAKE EDG. 5b | 1305+/-70BP |
| PERK4 | WOOD | 2.1 | PEAT | SKIPANON FP | 3110+/-70BP |
| PAUL1 | PALEOSOL | 2.5 | RECENT SND. | ON 2 DUNE | 160+/-50BP |
| RANK4 | PEAT | 8.2 | PEATY SILT | SKIPANON FP | 4050+/-70BP |
| RANK7 | PEAT | 4.5 | PEATY SITL | SKIPANON FP | 3070+/-65BP |
| SHAM1 | SHELL | 0.8 | SAND | ARCH. SITE 4b | 2110+/-50BP |
| SILM1 | WOOD/PEAT | 1.7 | PEAK | WETLAND E5e | 2560+/-60BP |
| 30CLT67 | CHARCOAL | 0.6 | RECENT SND. | ON 3A DUNE | 1250+/-50 |
| PALMR | CHARCOAL | 0.6 | MIDDEN | EAST OF 5b | 3650+/-100 |
CONCLUSION
The sequence of 17 approximately parallel dune ridges, dune breaches, wetlands and lakes reflect a complex shoreline history. The 21 km. (13 mile) modern foredune, and the approximately 17 km. (11 mile) foredune that existed prior to the construction of the south jetty of the Columbia River are long, but have not attained the height of the prehistoric dunes. The reasons for the major difference would appear to be time for development and sand supply, as well as the 3rd dune sequence having the modern dune overlay. The 3rd dune sequence has individual dunes that are also very wide. Ongoing research is establishing the extent of modern dune activity burial of the 3rd dune sequence, and field investigations have established windward side burial of the 3a and 3a dunes as deep as 8 meters (26 feet). The wide 3c and 3d bench dunes occur primarily in the central part of the Clatsop Plains in the Camp Rilea area, and may partially own their origin to modern dune filling as shown in (Figure 4) and human filling in of the area between the dune ridges to create flat ground. There are numerous dune breaches, particularly in the 3rd dune sequence. The dunes in the 4th dune sequence tend to occur very close together and are best expressed in the southern Surf Pines to Gearhart area. The oldest of the parallel ridges are the 5th dune sequence. These dunes have a wider spacing than the 4th dune sequence, and have considerable variation in height. These are the oldest dunes and possibly the most familiar to the public, because U. S. Hwy. 101 along the Camp Rilea area are built on top of the 5b dune in the sequence. Radio carbon dates are shown for the Clatsop Plains. The dates represent a variety of surface and subsurface conditions, and the interpretation of the dates is part of ongoing investigations. However the back date of about 5000 yrs. BP establishes that barrier beach progradation and dune development started after that time. Ongoing research is directed toward establishing the time sequence for all dune groupings, the reason for the groupings, modern dune overlay volumes, the reason for the dune breaches and the relationship of dune ridges to subduction zone earthquake events. .
ACKNOWLEDGEMENT
The United States Geological Survey Coastal and Marine Geology Program supported this work.
REFERENCES
Cooper, W. S. 1958. Coastal sand dunes of Oregon and Washington. GSA Memoir 72. New York, N. Y.
Peterson, C. D., Gelfenbaum, G.R. Jol, H. M., Phipps, J. B., Reckendorf, F. F., Twichell, D. C., Vanderburg, S., and Woxell, L. 1999. Great earthquake abundant sand, and high wave energy in the Columbia River Cell, USA. Proceedings from the Coastal Sediments 99 Conference, of the American Society of Civil Engineers. June 21-23, 1999, Hauppauge, Long Island, New York.
Rankin, D. 1983. Holocene geologic history of the Clatsop Plains foredune ridge complex. M.S. thesis. Portland State University, Portland, OR.
Reckendorf, F. 1975. Beaches and dunes of the Oregon Coast. USDA, Soil Conservation Service, Salem, OR.
Reckendorf, F., Leach, D., Baum, D., and Carlson, J. 1985. Stabilization of sand dunes in Oregon. Agricultural History 59:260-268.
Reckendorf, F. 1995. Clatsop Plains Oregon, Construction Setback Dunes Study. Reckendorf and Associates, Salem OR.
Woxell, L. 1998. Prehistoric beach accretion rates and long-term response to sediment depletion in the Columbia River Littoral System. M. S. Thesis. Portland State University, Portland, OR 206pp.