Bradley Lake, Coos County, Oregon

Bradley Lake location map

Location 43° 03' 33" N; 124° 25' 52" W
Surface elevation (m) 5
Watershed Area (ha) 646
Surface Area (ha) 9.4
Volume (m3) 655,963
Maximum Depth (m) 10.2
Mean Depth (m) 7.1
Shoreline length (km) 1.6
Trophic State Mesotrophic to Eutrophic

Introduction

Bradley Lake is a small natural lake located on the central Oregon coast four miles south of the City of Bandon (figure 1). The lake was formed during the Pleistocene Epoch by the impoundment of China Creek by migrating sand dunes, a common lake formation mode on the Oregon Coast (Cooper 1958). Several vacation and permanent homes line the forested shores of the lake. Residents and visitors boat, swim and fish in Bradley Lake. A public boat launch is maintained at the south corner and the lake is regularly stocked with rainbow trout.

Figure 1.

Figure 1, Bradley Lake watershed

Watershed Characteristics

The surface area of Bradley Lake covers only a small percentage of its total watershed area, 1.4% (table 1). The total watershed area is 646 hectares. Conifer and hardwood forests cover nearly 50%, urban-agriculture over 30%, and non-forested vegetation almost 20%. Sand dune areas at the western end of the lake cover less than 1% of the watershed area. The urban-agriculture areas are a mixture of housing developments, rangeland, farmland, and cranberry bogs. Non-forested vegetation areas mainly consist of marshland. Rainfall is averages about ten inched less per year than in areas to the north and south. Road density is high in comparison to other Oregon coast watersheds.

Table 1.

Table 1

Elevations range from five meters at the outlet of the lake to 223 meters in the foothills of the Coast Range. The western half of the watershed is located on the coastal plain and has low relief while the eastern half rises abruptly into the hills. The mean slope across the entire watershed is 5.2%.

Morphometry

Bradley Lake has one main basin with two small bays extending to the northeast and southeast (figure 2). The shoreline falls away steeply around most of the lake. Although the maximum depth measured is only 10.2 meters, the average depth is 7.1 meters. Shallow areas are located in the two eastern arms. A small ten-foot wide winding channel connects the main body of the lake with the boat launch in the southeast corner.

Figure 2.

Figure 2

Water Quality

Several water quality parameters were measured in Bradley Lake during the summer of 2001. A summary is provided in table 2. Bradley 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 but range into the eutrophic range.

Table 2.

Table 2

Bradley Lake was completely mixed during April of 2001 and stratified during July and September of 2001 at 3.0 and 3.9 meters respectively. The shallow depth of stratification is due in part to the high color, reported as tannic acid equivalents, of the water. The color prevents much penetration of light energy into the water column and this lack of energy allows shallow stratification. The color or tannic acids are most likely a result of humic compounds transported to the lake from wetlands and cranberry bogs within the watershed.

Thick beds of the non-native invasive macrophyte Egaria densa cover much of the sediment in shallow areas of the lake. Other species present include Typha latifolia, Lemna minor, Nuphar lutem, Potomogeton natans, Potentilla paulstris, Oenanthe sarmentosa, Myriophyllum spp., Juncus spp., and Polygonium spp.

Bradley Lake 2001 Data

APRIL 14:

Bradley Lake April 14, 2001 data

JULY 20:

Bradley Lake July 20, 2001 data

SEPT. 19:

Bradley Lake Sept. 19, 2001 data