Oregon Lake Inventory:
Garrison Lake, Curry County, Oregon

Introduction

In April 2002 the spit separating Garrison Lake from the Pacific Ocean was breached through either intentionally or through natural causes. Articles from several newspapers describing the breach have been reprinted on the Oregon Lakes Association website. Click on the Garrison Lake link when you get to the Gallery page.

Garrison Lake is a shallow coastal lake located within the city of Port Orford. The lake is labeled Garrison Lagoon on some older maps, but Garrison Lake has been officially adopted. It was named for John B. Garrison, a pioneer settler who was with Captain William Tichenor when he founded the town of Port Orford in 1851. The geologic history of Garrison Lake is typical of Oregon coast sand dune lakes; a rise in sea level inundated a coastal valley, migrating sand dunes blocked the mouth, and freshwater ponded behind the dunes (Cooper 1958). Recent events, however, make Garrison Lake unique among Oregon coastal sand dune lakes. During the winter of 1997/1998 a strong El Nino event breached the sand spit that separates the lake from the Pacific Ocean. The resulting saltwater intrusion has created meromixis within the lake. Meromixis is a salinity induced density stratification that is very stable. The lower stratified layer, the monomolimnion, is nearly as saline as the ocean (do comparison) and consequently very dense. The upper layer, the mixolimnion, is only slightly salty and consequently less dense than the monomolimnion. These layers do not mix with each other due to the density difference. During the summer of 2001, the monomolimnion was below approximately four meters.

Figure 1.

Fresh surface water enters the lake from two small, intermittent streams at the northeast arms. Surface outflow is at the southwest corner where a small stream follows a twisting course to the ocean. Generally there is no flow in this stream during the summer when the lake level is low. Water levels are reported to fluctuate up to one and a half feet throughout the season.

Garrison Lake is used for boating, fishing, and swimming. In the past the lake has produced good yields of stocked rainbow and native cutthoat trout. Prior to 1992, the lake was used for disposal of treated wastewater effluent generated by the Port Orford’s Publicly Owned Treatment Works (POTW). Concerned about eutrophication and degradation of the lake’s water quality, the Oregon Department of Environmental Quality (DEQ) issued an NPDES permit in 1987 with a requirement to relocate the effluent discharge from the lake to an alternative approved site. In 1992 a "dune disposal system" was permitted and constructed on the dune separating Garrison Lake from the ocean. The storm of 1997/1998 damaged the system. Currently treated effluent discharges into the sand on the westerly margin of the dunes. Several alternatives for remediation of the situation are on the table including ocean and Garrison Lake outlet discharge. More detail about Garrison Lake’s POTW history and remediation alternatives are available from SHN Consulting Engineers & Geologists, Inc. (http://www.shn-engr.com/portorford/Background.cfm).

Figure 2.

Watershed Characteristics

Garrison Lake covers a small portion of its total watershed area, only 6.4% (table 1). Business and residential sections cover over 50% of the watershed while forests cover 35%. Road density is the highest of any of the coastal lowlands lakes included in this study. The small remainder of the watershed is composed of non-forested vegetation (marshland) and barren areas (sand dunes). The forests contain mostly coniferous species including hemlock, spruce, fir and Port Orford cedar. Hardwoods such as madrone and x are also present. Most of the forest is young with diameters at breast height less than 20 inches.Both the slope and elevation range is low for coastal watersheds. Elevations range from near seal level to 122 meters. Watershed slope averages 4.5%. Precipitation is high on the low relief watershed.

Table 1.

Morphometry

Garrison Lake is the shallowest of the coastal lakes included in this study. Extensive areas are less than three meters deep (figure 3) and the entire lake averages 3.6 meters deep (table 2). The shallow depth and nutrient conditions allows for growth of a considerable amount of macrophytes (see the water quality discussion below). The deepest point measured in the lake, 9.1 meters, lies in the middle of the northern arm.

Table 2.

Water Quality

Several water quality parameters were measured in Garrison Lake during the summer of 2001. A summary is provided in table 2. Garrison’s trophic state based on both Secchi disc depth and total phosphorus concentration ranged from index values of 35 to 45, or from oligotrophic to mesotrophic. Water quality indices values are similar to, but slightly lower than those calculated from data collected by the Oregon Department of Environmental Quality and Portland State University between 1981 and 1996 (figure 3).

Figure 3.

Oxygen was depleted below the chemolimnion during the two visits when thermal stratification was present. Figure 4 presents vertical profiles of temperature, oxygen and other parameters.

Macrophyte density is particularly high and troublesome in Garrison Lake. The non-native species Elodea densa was reported to be dense in 1988 and 1989 (Geiger et al. 1990) however, none were encountered in this study during the summer of 2001. This may be a result of the saltwater intrusion that occurred during the winter of 1997/1998. Ruppia maritime, a species tolerant of brackish water, was very dense in 2001. Potomogeton crispus, Potomogeton richarsonii, Potomogeton natans, Myriophyllum sibiricum and Chara spp. were also present.

Garrison Lake, 2001 Data

APRIL 24

JULY 20

SEPT. 19

Resources

In Preparation