Oregon Lake Inventory:
Clear Lake, Douglas County, Oregon

Introduction

Clear Lake is a large, deep natural lake located near the Oregon coast (figure 1). It serves as the primary municipal water supply for the City of Reedsport located five miles to the north. The city has restricted public access to the lake and watershed in an effort to protect this resource. The lake is well known, however, because of its location next to the heavily traveled Coast Highway. Clear Lake heads a chain of lakes that includes Lake Edna, Eel Lake and the Tenmile Lakes. The chain was formed during the Pleistocene epoch through subduction of coastal areas followed by ponding of streambeds by migrating sand dunes (Cooper 1958). Although this type of lake origin is common on the Oregon coast, the lake’s elevation of near 70 meters is unusually high.

Figure 1. Lake watershed

Clear Lake receives surface inflow from several intermittent streams. Outflow is south through the chain of lakes, Tenmile Creek and into the Pacific Ocean. The USGS reports that seepage occurs into unconsolidated sediments along the south and southwest margins of the lake (Rinella et al 1980).

Watershed Characteristics

Forests and water are the major components of Clear Lake’s small watershed covering approximately 90% of the total area (table 1). Conifer forest comprises 66% of the total watershed area and the surface area of the lake 23 % of the watershed area. The remaining ten percent consists of sand dunes, non-forested vegetation, and roads. Road density is 22 meters per hectare, similar to other Coast Range watersheds. Some of the roads lead to a clear-cut logging area which occurred within the watershed during the winter and spring of 2001.

Table 1. Lake watershed characteristics

Average slope is 15% across the watershed. As noted earlier, the elevation of the lake is high when compared to other dune formed lakes on the Oregon coast. Elevations range from 70 meters at the outlet to 225 meters in the Coast Range. Mean elevation is 109 meters. The Coast Range and sand dune slopes in the watershed are steep, averaging 15%. The mean annual precipitation of 74 inches per year is typical of the rainy Oregon coast.

Morphometry

Clear Lake has the characteristic dendritic shape of a dune formed lakes. Due to a limited boat use on Clear Lake and the windy nature of the area, an updated bathymetric map could not be completed. The U.S. Geological Survey did, however, complete a bathymetric map from data collected October 1977 (figure 2). The lake averages 16.5 meters deep and has a maximum depth of 36 meters. The shoreline drops very steeply with few exceptions. The tips of each of the arms and some bays have areas shallow enough to support limited rooted macrophyte growth.

Water Quality

Several water quality parameters were measured in Clear Lake during the summer of 2001 (methods link). A summary is provided in table 2. The lake’s trophic state based on Secchi disc depth, chlorophyll a concentration, and total phosphorus concentration ranged from index values of 28 to 71. This is an extremely wide range spanning oligotrophic to eutrophic values. The highest values were observed on April 18, 2001. Samples collected on that day were during the midst of an intense phytoplankton bloom. Chlorophyll a was 63 µg/l and Secchi disk depth was 3 meters, both values very poor for a drinking water supply lake. Trophic state values calculated from July and September data ranged from 28 to 37, all within the oligotrophic range. City of Reedsport water supply managers report that during the spring of 2001 Clear Lake’s water quality was as poor as they had ever seen it. The poor water quality observed in April may reflect an input of nutrients from new clear-cut areas in the watershed. This assertion has not been verified, however. What is evident from the existing data is that Clear Lake’s water quality is very sensitive to change.

Table 2. Water quality table

Thermal stratification was present during the July and September visits. The mixed depth was over ten meters during both visits (table 2). Oxygen was depleted in the lowest portions of the hypolimnion during the September visit. Vertical profiles of temperature, oxygen and other parameters are provided below.

The morphometry, water quality, and sediment quality of Clear Lake were not sufficient to support dense beds of macrophytes. The most common macrophyte encountered was a species of Isoetes. Sparganium emersum, Vallisneria Americana, and Potomogetum spp. were also present.

Clear Lake South, 2001 Data

APRIL 19

JULY 18

SEPT. 17

Resources

Christensen, R., and B. Rosenthal. 1982. North Florence dunal aquifer study. Lane Council of Governments, Eugene, OR. 153 pp.

Cooper, W. S. 1958. Coastal Sand Dunes of Oregon and Washington. N.Y. Geol. Soc. Am. Memoir 72. New York, NY. 169 pp.