Morphology and Evolution of Salmonid Habitats in a Recently Deglaciated River Basin, Washington State, USA

Abstract

Morphology and distribution of salmonid habitats were related to the geomorphology of a river basin at three spatial scales including reach (10 2 –10 3 m 2 ), subbasin (2–26 km 2 ), and the watershed (240 km 2 ). Stream reaches on a young fluvial terrace (1700 yr old) adjacent to the main river contain the most extensive areas of rearing and spawning habitats. In tributary subbasins, the area of spawning habitat varies according to discharge rates and channel gradients. The most extensive salmonid habitats are located along wide glacial deposits in geologically unconstrained areas of the main valley floor. During the early Holocene (~10 000 – 12 000 years before present (B.P.)), the recently deglaciated watershed of the South Fork Stillaguamish River was extremely erosive and vegetated by alpine forest. Fish habitats then were less suitable for salmonid rearing and spawning. A much lower erosion rate after 8000 yr B.P., and the advent of old growth conifer forests after 6000 yr B.P., indicates that stream habitats attained their present-day morphology between 8000 and 6000 yr ago. Although habitats increased in quality with increasing watershed stability and evolution of forests, they decreased in quantity after 7000 yr B.P as landforms changed because of continuous river incision into glacial deposits.