The name comes from a Lummi word meaning loud water. Long before it meant a reservoir, the south end of the lake held a Coast Salish village occupied by the Saquantch; around 1800 the Lummi pushed them out, and by the 1850s the first settlers of European descent had arrived. One early private claim on the lake reportedly changed hands for eight dollars. The lake's only island is a three-acre scrap of rock and fir called Reveille Island, now owned by Camp Firwood, where pictographs and a carved stone bowl in animal form suggest it was a ceremonial place. There is no plaque explaining it, no interpretive trail. A summer camp keeps a small island that people were doing something important on a very long time ago.

Lake Whatcom is really three lakes that happen to touch. Basin 1 at the north end, the Silver Beach Basin, bottoms out around 100 feet and is ringed by houses. Basin 2, the Geneva Basin in the middle, is the shallow one - 40 to 60 feet at most - and it is where Bellingham's intake sits. Basin 3 runs south and deep, dropping to 328 feet and holding an estimated 96 percent of everything in the lake. Submerged sills divide the three, and water moves between them slowly, which is why the lake behaves less like a single body than like a set of tanks connected by narrow doors. The whole thing stretches about ten miles and rarely exceeds a mile across.

Lake Whatcom is monomictic: it mixes top to bottom just once annually. From late spring into mid-autumn the sun warms an upper layer that floats on colder water below, with a sharp temperature drop between them. Oxygen from the air barely reaches the bottom layer, and sediments consume what little gets there, so the deep water of Basin 3 goes nearly anoxic by summer's end. Then the surface cools, the layers collapse into each other, and the lake turns over. This is normal limnology. What complicates it is nutrient loading - and Lake Whatcom gets more phosphorus than a lake this shape can absorb.

The EPA added Lake Whatcom to its 303(d) list of impaired waters in 1998 for low dissolved oxygen, which traces directly back to phosphorus. Most of it arrives diffusely: stormwater running off roofs and roads, lawns and gardens, aging septic systems in the watershed. In 2005 phosphorus fertilizers were banned around the lake. Bellingham and Whatcom County restrict land clearing from October 1 through May 31, the wet months, to keep bare soil from washing downhill. A 2008 state Department of Ecology analysis concluded that hitting acceptable oxygen levels would require returning hard, water-shedding surfaces to their pre-1988 extent - which is to say, unbuilding roughly two decades of development. By 2014 the watershed counted 6,877 dwelling units, some 2,566 of them in the Sudden Valley neighborhood alone, with thousands more lots still buildable. The city and private owners have put about 2,519 acres of the watershed's 31,127 under conservation easement. The arithmetic is not subtle.

Nearly 78 percent of the lake's outflow leaves through Whatcom Creek, past control gates at the head of the creek in Whatcom Falls Park where the city meters the level - balancing storage, drinking water demand, and enough flow to keep threatened Chinook salmon moving. Water also arrives by tunnel: since 1962 a diversion has carried water from the Middle Fork of the Nooksack River into the lake, built to feed a paper mill on the Bellingham waterfront. The hills around the lake were logged out by the end of the 1800s, and coal was mined nearby until the Whatcom Mining Company shut in 1919. In 1946 J.H. Bloedel gave the city 12.5 acres that became Bloedel Donovan Park. Thirteen fish species live here now - kokanee, coastal cutthroat, sculpin and stickleback among the natives; bass, perch and bullhead among the arrivals, some stocked and some dumped from buckets. Boats still crowd the north end on July weekends. So does the water everybody drinks.