Mercer Lake (Lane)

Reachcode: 17100205000491 | Area: 350.1 acres | Shoreline: 9.0 mi | View on Interactive Map

(From Atlas of Oregon Lakes, Johnson et al. 1985). Mercer Lake is a mid-sized lake of 359 acres on the central Oregon coast. It was named in the 1880s for a government surveyor, George Mercer, who surveyed the township line that passes through the lake. The former Indian name for the lake was Kow-y-ich, meaning simply "the place of the lake". The geologic origin of Mercer Lake can be traced to events associated with sea level fluctuations during and since the Pleistocene ice ages. At one time a stream, fed by several tributaries rising in the coastal mountains, flowed across a marine terrace north of Florence to reach the Pacific Ocean. Submergence of the entire coastal strip due to the post-glacial rise in sea level drowned the lower reach of the stream. Subsequently, sand dunes blocked the mouth, impounding the flow of the stream to form Mercer Lake. A sand barrier of this type develops for two reasons, deposition by the stream itself whose gradient has been reduced by submergence of the mouth, and migrating sand dunes brought about by longshore currents, wave action, and wind action. Several other nearby lakes were formed in much the same manner at about the same time; for example, Sutton Lake and the chain of lakes immediately to the south of Mercer Lake -- Collard, Clear, and Munsel Lakes (Cooper 1958).

Mercer Lake thus occupies a former stream valley and has the characteristic dendritic or branching shape of impoundments, whether natural or artificial, that occur in stream valleys. The shoreline is long and irregular. Average water surface elevation is 32 feet above sea level, while the bottom of the lake is below sea level (maximum depth = 38 feet). The lake basin is steep-sided and has a fairly flat bottom due to the accumulation of sediments. Material from the steep slopes around the shoreline frequently slumps into the water. Deposition of material by inflowing streams has filled in much of the area of the former water-filled valleys of Bailey, Levage and Dahlia Creeks, gradually reducing the size of the lake. The perennial outlet is into Sutton Creek Marsh which drains into Sutton Lake.

The drainage basin is quite small and is covered primarily with a coniferous forest on steep slopes. Much of it is land within the Siuslaw National Forest, although there are many private land holdings within the broader National Forest boundaries. The shoreline is about 95 percent private land on which there is a resort and numerous homes. A county road leads from the Coast Highway to the south side of the lake. The road extends up the east side to a public boat ramp at the end of the northeast arm on Forest Service land. Mercer Lake has never been fished very heavily, but it provides excellent angling for trout from early spring well into the summer. It also contains some good-sized largemouth bass and yellow perch. Angling is difficult from the brushy shoreline and boats are a must.

Mercer Lake is an excellent example of a beautiful coastal lake in which the water quality has deteriorated noticeably in response to cultural activities. Concentrations of major inorganic ions in the water are low and alkalinity is low. Macrophytes are not a serious problem because of the steep shoreline, although Elodea is common in some shallow areas. However, the concentration of chlorophyl is above average for coastal lakes and the water transparency below average. Development in the basin proceeded at a rapid rate in the 1970s, particularly along the shoreline. Sixty percent of the shoreline is now developed and many of the homes are at the water's edge. Much of the shoreline has been made accessible by paved roads. Previously published Secchi-disk values indicate a trend of advancing eutrophication. Kruse (1962) reported values ranging from 11.8 to 12.8 feet (3.6 to 3.9 meters), and later Kavanaugh (1973) measured values ranging from 8.5 to as much as 26.6 feet (2.6 to 8.1 meters). Lane County measured values ranging from 6.6 to 14.8 feet (2 to 4.5 meters) in the summers of 1978 and 1979 (Bryant et al 1979). In the survey reported on in this volume, values were lower: 8.2 feet (2.5 meters) on 7/18/81 and 6.6 feet (2.0 meters) on 11/19/82.

Incipient signs of eutrophication can be seen in the floating and emergent stands of macrophytes in the many small bays along the shore. Cattails, bullrushes, sedges and water lilies grow densely in these shoal areas. Further evidence that development has influenced water quality in the lake can be cited. Bryant et al (1979) found higher nutrient concentrations in the more heavily developed "arm" of the lake than in other arms, and they believe that the lake, in general, has become more productive in recent years. There is much land for sale adjacent to Mercer Lake; the future will undoubtedly include increased development and water quality should be monitored to detect perturbations resulting from this development. Bryant et al (1979) recommend: (1) provide water quality analysis every three to five years; (2) develop regulations to prevent erosion; and (3) support citizen group efforts for preservation. It should be noted that these recommendations apply to all of the coastal lakes studied by Bryant etal (Sutton, Collard, Clear, Woahink, and Siltcoos), but especially to Mercer Lake which had the highest concentrations of nitrogen and phosphorus.

Data collected in this survey show that nitrate is especially high in Mercer Lake (1.02 ppm in May; 0.23 ppm in November). Values for trophic state indicators were at the high end of mesotrophy (Secchi index 47, chlorophyl index 61, and phosphorus index 53). The lake is stratified for at least part of the year, the hypolimnion being low or lacking in oxygen during the summer and fall months. The thermocline varies from year to year, but generally lies at 23 to 30 feet (7 to 9 meters). Winter overturns and complete circulation have been recorded in December and January. Kavanaugh (1973) determined that nitrate was replenished during fall overturn.

An analysis of available phytoplankton data presents a somewhat inconsistent picture with regards to trophic state. A net phytoplankton sample collected on 8/29/74 was predominantly Tabellaria fenestrata, an alga generally assumed to indicate oligotrophic conditions. The sample collected on 7/18/81 was dominated (by weight) by Anabaena sp. and Fragilaria crotonensis, which are most often associated with eutrophic lakes. Tabellaria fenestrata was not found in any of the samples collected in 1981 and 1982 for this study. Another phytoplankton sample collected on 10/30/72 was dominated by Melosira granulata, an alga also not found in the samples of 1981 and 1982. These changes in phytoplankton support other evidence of advancing eutrophication in Mercer Lake. The nannoplankton samples collected in 1981 and 1982 for this study are composed of mesotrophic to eutrophic species, namely Asterionella formosa, Anabaena sp., Chroomonas sp., and Fragilaria crotonensis. When water temperatures are cooler in spring and fall, Dinobryon and Cyclotella stelligera appear and indicate a lower trophic state; but they are less abundant than the eutrophic algae.

In summary, Mercer Lake is an excellent example of a coastal lake which has begun to deteriorate in response to cultural influence. In spite of the fact that much of the lake is deeper than 30 feet, as are some oligotrophic coastal lakes, it has a higher than average trophic state for Oregon coastal lakes. The current trophic state for Mercer Lake is mesotrophic, but is approaching the eutrophic classification. 

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No mussel data available.

The list of plants below includes results of aquatic plant surveys conducted by the Center for Lakes and Reservoirs as well as aquatic invasive plant species detections that have been reported to iMap Invasives: an online, GIS-based invasive species reporting and querying tool.

Plants listed in the table below are categorized as native to Oregon, on the Oregon Department of Agriculture’s (ODA’s) Noxious Weed List, on the Federal Noxious Weed List, or non-native but not listed as noxious. Federal Noxious Weed List plants are plants determined by USDA to be serious threats to U.S. agriculture, irrigation, navigation, public health or the environment (7 C.F.R. 360.200). The ODA Noxious Weed categories are:

ODA Class A - weeds either unknown or with small enough infestations to make eradication or containment possible; targeted for eradication or intensive control.

ODA Class B - regionally abundant weeds (may have limited distribution in some counties); targeted for local/regional control on case-by-case basis.

Download the complete dataset as a CSV

Date Species Status Source
Aug. 15, 2003 Egeria densa (South American waterweed, Brazilian elodea) Non-native ODA Class B IMAP