Grass Seed Agriculture and Invertebrate Communities of Seasonal Wetlands in the Southern Willamette Valley

Introduction In the lowlands of the upper Willamette River Valley, Oregon, winter flooding is an annual phenomenon driven by the seasonal precipitation regime of the Pacific Northwest (Hamlet and Lettenmaier 1999). Predictable inundation of river floodplains and expansion of stream networks like these are major processes influencing entire aquatic communities (which include fish, frogs, salamanders, insects, clams, snails, and crustaceans-such as crayfish and small shrimp like organisms) (Tockner and Stanford 2002). During the past 150 years in the Willamette River Valley the cumulative effects of navigation improvement and flood control projects, agricultural activities and urban development have altered the hydrology of the system, removed miles of side channels, and eliminated acres of floodplain that were regularly connected to the main river channel (Boag 1992; Hulse et al. 2002). Flood control and improved drainage has been achieved through channel straightening, wetland filling, dike construction and ditch development (Benner and Sedell 1997). Channel straightening alone eliminated meanders and secondary channels in the Willamette River and reduced the length of its main stem. Along a portion of the Willamette River, between the McKenzie River confluence and the city of Albany, the main channel is approximately 45% to 50% shorter than it was in 1850 (Benner and Sedell 1997; Hulse et al. 2002). Also, lowland floodplains have been partly drained through ditches and underground tile systems for agriculture. These floodplains sustain 95% of Oregon’s grass seed production, and represent approximately 50% of the tillable land (some 2,000 sq km) in the entire Willamette River Valley (Gohlke et al. 1999).