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Water Quantity and Quality

Willamette Basin Rivers and Streams

Beginning in the Calapooia Range of the Cascade Mountains, the Willamette River drains a basin lying between the Cascades and the Coast Range approximately 180 miles (290 km.) long and 100 miles (161 km.) wide, encompassing 11, 478 square miles (29,728 sq. km.)

The interaction of water and land in a large-scale hydrological system creates a dendritic drainage pattern, emulating the branching pattern of deciduous trees, or the vascular system of leaves. Map 4 depicts stream order, a way of describing the pattern of connections within the hydrological network. Stream size, in terms of flow and area drained, generally increases with increasing stream order. The best available map sources for streams do not include smallest perennial streams or ephemeral ones. Field work suggests that these small streams are ecologically important and numerous. Their omission significantly understates the actual length of the stream network and causes stream order to be under reported by approximately two orders.

Streams on the west side of the basin tend to be more sluggish, with lower base and minimum discharges and higher temperatures than streams on the east side of the basin where the influence of geology and snowpack make for more uniform temperature and flow rates.

Excerpted with permission from Branscomb, J. Goicochea, M. Richmond. (2002). Stream Network. In D. Hulse, S. Gregory, and J. Baker (Eds.). Willamette River Basin planning atlas (PDF), 2nd. Edition, (p.16.). Corvallis: Oregon State University Press