CONCLUSIONS

The largest historic flood in the San Juan River occurred on October 6, 1911, with a discharge of 4,200 m³/s (148,000 ft³/s) and a recurrence interval of about 150 years. This flood appears to have removed all older flood evidence from San Juan Canyon below its peak stage. No flood evidence was found higher than the 1911 flood deposits despite ideal settings for preservation, suggesting that the 1911 flood may represent the largest flood on the San Juan River for a much longer time period than 1880-2001. Wide, vegetation-free channel conditions existed before the 1911 flood and afterwards until the last half of the 20th century, suggesting that this flood alone did not significantly affect the distribution of riparian communities. Channel narrowing occurred after 1941, as it has on other rivers in the region. Native and nonnative riparian vegetation have encroached on the previously wide channel in the alluvial reach and in previously barren parts of the bedrock canyon. The increase in riparian vegetation has accelerated in recent decades. Changes in flood frequency and riparian vegetation are affected both by climatic fluctuations and flow regulation. The increase in riparian vegetation, particularly in the last 30 years, is associated with a regional increase in precipitation, particularly during winter. Unlike other rivers, winter floods have not increased, in part because of the operation of Navajo Reservoir. Construction of Navajo Dam and Reservoir has affected the San Juan River in complex ways that are not fully addressed in this study, including the attenuation of flood peaks caused by storms at higher elevations in the basin, storage of sediment generated in the higher elevations of the basin, and changes in seasonal flow patterns, all of which could have influenced the increase in riparian vegetation. Because flood waters mostly are generated at the higher elevations and sediment mostly is generated at lower elevations in the drainage, the channel narrowing is consistent with a river with decreased flow competence, or ability to transport sediment, but with a continued high sediment yield from its tributaries.

Acknowledgments. We offer special thanks to the River Office staff of the Monticello Field Office, Bureau of Land Management. Dominic Oldershaw matched many of the photographs. Peter Griffiths and Jessica Young helped with the illustrations. Additional thanks to the many field assistants who endured freezing rain, blistering heat, flash floods, and all of the other joys of working in such a beautiful canyon.