Dynamic patch mosaics and channel movement in an unconfined river valley of the Olympic Mountains

Abstract

Summary1. River valleys resemble dynamic mosaics, composed of patches which are natural, transient features of the land surface produced by the joint action of a river and successional processes over years to centuries. They simultaneously regulate and reflect the distribution of stream energy and exchanges of sediment, wood and particulate organic matter between riparian and aquatic environments.2. We determined the structure, composition, dynamics and origin of seven patch types at the reach scale in the Queets River valley in the temperate coastal forests of the Olympic Mountains, Washington (U.S.A.). Patch types included: (1) primary and (2) secondary channels; (3) pioneer bars; (4) developing and (5) established floodplains; and (6) transitional and (7) mature fluvial terraces.3. Lateral channel movements strongly shape patch distribution, structure and dynamics. The primary channel moved laterally 13 m year−1, on average from 1939 to 2002, but was highly variable among locations and over time. Mean lateral movement rates ranged from 1 to 59 m year−1and moving averages (2 km) ranged from 3 to 28 m year−1throughout the valley.4. Each patch type exhibited characteristic vegetation, soil and accumulations of large wood. Pioneer bars contained peak stem density (69 778 stems ha−1) and volume of large wood (289 m3 ha−1). Mature fluvial terraces contained the highest mean stem (1739 m3 ha−1) and canopy volume (158 587 m3 ha−1). These patches also contained the most soil nitrogen (537 kg ha−1) and carbon (5972 kg ha−1).5. Patch half‐life (the time required for half of the existing patches to be eroded) ranged from 21 to 401 years among forested patch types. Erosion rates were highest in pioneer bars (2.3% year−1) and developing floodplains (3.3% year−1), compared with only 0.17% year−1in mature fluvial terraces. New forests formed continually, as pioneering vegetation colonised 50% of the channel system within 18 years, often unsuccessfully.6. In the Queets River, the structure, composition, and dynamics of the patchy riparian forest depends on the interplay between channel movements and biophysical feedbacks between large wood, living vegetation and geomorphic processes. The cycle of patch development perpetuates a shifting‐mosaic of habitats within the river valley capable of supporting diverse biotic assemblages.