The Tolt River sub-basin is one of the largest in the watershed. At 100 mi2 it is larger than the South Fork Snoqualmie and roughly the same size as the North Fork Snoqualmie. The sub-basin is significant and unique on a number of fronts. The South Fork Tolt reservoir is a core component of the regional water supply system and the sub-basin is one of the most significant spawning areas for Snoqualmie Chinook salmon and for other salmonids. Also, the Tolt River is home to the only known naturally occurring spawning population of summer steelhead in the watershed. More than 90% of the basin is forested under both public and private ownership.

The Lower Tolt is a very dynamic river with a very active channel migration zone. King County and the City of Seattle have acquired a large number of residential parcels from private property owners who have suffered repetitive property damage during floods. Parcels acquired by King County are collectively managed as the Tolt River Natural Area, a discontinuous collection of parcels totaling 240 acres that are mostly within the Tolt River’s 100-year floodplain.

Moss Lake Natural Area, located along a right-bank tributary to the Lower Tolt, is a King County Department of Natural Resources and Parks (DNRP) Ecological Land. The site is located 3.5 miles northeast of Carnation. This 372-acre site contains high-quality wetland and forested upland habitats. An extensive Class 1 wetland complex encompasses a large sphagnum bog, beaver dams, open water and forested wetland. The lake and associated bog and wetland comprise a rare habitat type in King County, and the fact that the site is relatively unaltered makes it a unique resource. In addition, the surrounding upland forest provides valuable wildlife habitat. Several King County species of concern - including bald eagle, Vaux’s swift, red-tailed hawk, pileated woodpecker, bandtailed pigeon, western toad and Beller’s ground beetle – are known to utilize the area.