Geology

Beneath the surface of the watershed lie the sedimentary rocks of the Chuckanut Formation with a metamorphic rock called phyllite and glacially derived sand and gravel being exposed at the far south end of the lake. The Chuckanut Formation, often referred to as Chuckanut Sandstone, extends from the Cascade Range to Lummi Island and is actually a group of rocks that includes layers of sandstone, conglomerate, shale, and coal. 

The same tectonic forces that shaped the Cascade Range also tightly folded the layers of the Chuckanut Formation to create the Chuckanut Mountains including the hills around present day Lake Whatcom. More resistant layers, typically sandstone, tend to form ridges or high points while weaker rocks, such as shale, will form low points.

During the last Ice Age, glacial ice covered western Whatcom County to depths greater than 5,500 feet. This tremendous volume of ice scoured the underlying rock. The current lake bathymetry, or underwater topography, was created as the glacier advanced and retreated multiple times. It scoured the less resistant rock, while leaving the two sills of resistant material that now divide the lake into its three distinct basins. The tremendous weight of the ice also depressed the land beneath it, much like a finger pushed into a balloon. As the ice retreated approximately 10,000 years ago, the weight was relieved and the land began to rebound.

After the ice was gone and as sea levels rose with water released from the melting glaciers, but before the land had fully rebounded, it is likely that Lake Whatcom formed a fjord that was directly connected to marine waters. There are a couple of things about the watershed that indicate this possible past connection with marine waters — fossils and fish. Marine fossils have been found in deposits near the northern portion of the lake. Kokanee, the land-locked form of sockeye salmon, are found in Lake Whatcom. These fish probably became isolated from their ocean-going counterparts as the land rebounded. As the land rebounded, the lake became higher than Bellingham Bay and natural barriers formed between the marine and lake system, such as the multiple waterfalls in Whatcom Falls Park.

The soils that are derived from the Chuckanut Formation and the steep topography of some portions of the watershed leave the area naturally prone to landslides. Land uses such as timber harvest, forest road building and usage, residential development, and utility installation and maintenance can all increase the natural rates and timing of sediment delivery, with associated effects on stream and lake water quality, fish and fish habitat, and ultimately human health and safety.