North Fork Reservoir (Clackamas)

Reachcode: 17090011005288 | Area: 218.5 acres | Shoreline: 4.5 mi | View on Interactive Map

(From Atlas of Oregon Lakes, Johnson et al. 1985) North Fork Reservoir is the largest of several impoundments operated by the Portland General Electric Company on the Clackamas River. It was formed by construction of a 207-foot high, concrete arch dam in 1958 about a mile below the junction of the North Fork Clackamas River with the main stem, and 29 miles upstream from the confluence with the Willamette River. The project produces power on a peaking schedule, and water level in the reservoir fluctuates about four feet daily. Storage capacity is close to 19,000 acre-feet at full pool, large enough to insure continuous turbine operation not only at its own powerhouse, but for those at Faraday and River Mill powerhouses downstream. Nearly two million acre-feet of water flow through the Clackamas River in an average year, which means that the retention time in the reservoir is very small. The narrow, deep reservoir stretches four miles into the western slope of the Cascades. It is an area of rugged topography, composed of older volcanic flows deeply cut by tributary streams. Coniferous forests cover the landscape in the 644 square mile drainage basin. This is mostly within the Mt. Hood National Forest, although the reservoir itself is outside of the national forest. Much of the shoreline is in private ownership, but is undeveloped. North Fork Reservoir is close to the Portland metropolitan area and receives heavy use for water-based recreation. It is very popular for boating, water-skiing, and fishing. It is not very productive for native fish but is frequently stocked with rainbow trout. Other species have entered from the river upstream and are caught at times. Boat launching areas are provided and a 16-acre park, Promontory Park, is operated by Portland General Electric. There is also a private concession with fishing and boating supplies at the upper end of the lake. Several forest service camps are located upstream. A 10-mile-per-hour speed limit is enforced on the upper two miles of the reservoir, and keeps speed boats from interfering with angling in the resort and park area. Water chemistry in North Fork Reservoir (concentrations of major ions, alkalinity, and conductivity) is typical of streams draining the west slope of the Cascades. The concentrations of chlorophyl and phosphorus are moderate, as is water transparency (Secchi disk depth = 14.4 feet; 4.4 meters), and indicate mesotrophic conditions. Water temperatures remain quite cool through the summer and the water column is saturated with oxygen. Phytoplankton densities are fairly low; the greenish cast to the water results from very fine particulate matter in the runoff rather than from biological growth.


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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.

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Date Species Status Source
Aug. 14, 2014 Typha latifolia (common cat-tail) Native CLR
Aug. 14, 2014 Elodea canadensis (common elodea, Canadian waterweed) Native CLR
Aug. 14, 2014 Lemna sp. (duckweed) Native CLR
Aug. 14, 2014 Bryophyte, aquatic (moss or liverwort) Native CLR
Aug. 14, 2014 Sparganium angustifolium (narrowleaf bur-reed) Native CLR
Aug. 14, 2014 Callitriche hermaphroditica (northern water-starwort) Native CLR
Aug. 14, 2014 Potamogeton pusillus (slender pondweed) Native CLR
Aug. 14, 2014 Nitella sp. (stonewort) Native CLR
Aug. 14, 2014 Ranunculus aquatilis (water-buttercup) Native CLR
Aug. 14, 2014 Schoenoplectus subterminalis (water clubrush) Native CLR
Sept. 13, 2011 Fontinalis sp. (aquatic moss, fontinalis moss) Native CLR
Sept. 13, 2011 Elodea canadensis (common elodea, Canadian waterweed) Native CLR
Sept. 13, 2011 Sphagnum sp. (peat moss) Native CLR
Sept. 13, 2011 Potamogeton pusillus (slender pondweed) Native CLR
Sept. 13, 2011 Nitella sp. (stonewort) Native CLR
No photos available.

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