Oregon Lake Inventory:
Eel Lake, Douglas and Coos Counties, Oregon

Introduction

Eel Lake is a large, deep coastal lake south of the mouth of the Umpqua River. Eel Lake, Clear Lake to the north, and Tenmile Lakes to the south owe their existence to the same series of geological events as described by Cooper (1958). Tenmile Creek was the mainstem of a stream network that drained this entire region. Eel Creek and its tributary Clear Creek were two sources of Tenmile Creek. During a time of coastal submergence that accompanied post-glacial warming, the entire system went through a period of readjustment. The main valley and the lower courses of its tributary valleys were inundated. A bar of sand dunes formed across the river mouth and the ponded valley thus became a large lake with branches extending up the tributaries. As submergence continued the depth of the lake increased and arms extended farther upstream. At the same time, dunes along the shore north of Tenmile Creek were advancing inland and eventually entered broadside into the valley of Clear Creek, impounding its headwaters and forming what is now Clear Lake. As drainage from Clear Lake flowed into the Eel Creek valley a delta began to form that developed into a sand flat blocking off the upper portion of the valley. The resultant ponding gave rise to Eel Lake. The same process proceeded downstream to form North and South Tenmile Lakes. The primary inflows to the present day lake include Clear Creek, which originates in Clear Lake and passes through Lake Edna, and two unnamed tributaries that flow into the heads of the east and west arms of the lake (figure 1).

Figure 1. Eel Lake watershed

The Lakeside Water District holds a small water right for municipal use of Eel Lake water, but the primary function of the lake is for recreation. Fishing is popular with native cutthroat trout, stocked rainbow trout, steelhead, coho salmon, smallmouth bass, and panfish present. A boat launch, picnicking, hiking trails and restrooms are provided at William M. Tugman State Park on the southwest shoreline.

Watershed Characteristics

Eel Lake is surrounded by steep timber covered slopes characteristic of the foothills of the Coast Range The majority of Eel Lake's watershed, 63%, consists of mainly coniferous forests (table 1). Most of the forest is relatively young. The remainder of the watershed includes non-forested vegetation (marshes and parkland), open water, and barren areas to the west (sand dunes). There are a few residential areas to the west of the lake, but most land is owned by logging interests. There are no homes on the lakeshore. Slopes are steep throughout the watershed. The mean slope is sixteen degrees, the highest among coastal lakes included in this survey. Elevations range from 16 to 228 meters above sea level. Rainfall averages 74 inches per year across the watershed, typical for the Oregon Coast Range area.

Table 1.

Morphometry

Nearly all the shoreline of Eel Lake drops precipitously to a flat bottom of over eighteen meters deep. A bathymetric map of Eel Lake is provided in Figure 3. The average depth is 12.5 meters (table 2). The deepest point measured of 20.9 meters lies in the middle of the southern section of the lake. Relief in the northern ends of both arms is not quite as dramatic. In these sections, extensive areas less than ten meters deep slope to a shoreline of marshes at the northern ends. Shoreline development, an index of the shape, is high as would be expected with a dendritic lake.

Figure 2. Eel Lake bathymetric map

Table 2. Eel Lake water quality

Water Quality

Several water quality parameters were measured in Eel Lake during the summer of 2001. 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 40 to 51. These values are within the mesotrophic range and slightly higher than values calculated from historical data collected by Portland State University and Oregon Department of Environmental Quality (Johnson et al. 1985). A plot of trophic state index values based on these data is provided in figure 4. These data are not sufficient to infer long-term trends or causal mechanisms, however, clear-cut logging activity in the watershed in 2001 may have impacted water quality.

Figure 3. Carlson's trophic state index

Eel Lake was thermally stratified during visits in July and September at depth of 6.8 and 7.9 meters respectively. Vertical profiles of temperature and other data collected with a water quality probe are plotted below. The entire water column was oxic during the April and July visits, but was anoxic at the deepest section during the September visit. Although anoxic layers are not typical of oligotrophic lakes, this trend does not appear to apply to oligotrophic lakes located in the Oregon coastal dunes.

Macrophyte beds are most extensive in the shallow northern arms and bays throughout the lake. Dense stands of the non-native invasive species Egeria densa cover much of the near shore sediments of the lake. Floating leaf species present include Nuphar luteum, and Brasenia schreberi. Emergent species include Typha augustifolia, Equisetum fluviatile and Sparganium augustifolia.

Eel Lake, 2001 Data

APRIL 18

JULY 19

SEPT. 18

Resources

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