Sources and distributions of sedimentary organic matter in the Columbia River drainage basin, Washington and Oregon1

Nonwoody angiosperm tissues and gymnosperm woods are the major types of vascular plant debris in bottom sediments from 16 sites within the Columbia River and its tributaries. In tributary sediments relative abundances of these types of tissue vary regionally and are consistent with drainage basin vegetation. Within the extensively dammed main river, vascular plant debris mixtures are more uniform, indicating downstream mixing of sedimentary material from reservoir to reservoir. Vascular plant tissues account for an average of a third of the total sedimentary organic matter at the 16 river sites. The remaining organic material has an atomic C:N of 10–15 and a stable carbon isotope composition similar to “average” land plants. Organic matter in sediments of the Columbia River system is generally enriched in nonwoody angiosperm tissues and lignin‐poor organic matter in comparison to Columbia River‐derived organic matter in modern marine sediments from the southern Washington continental shelf.