Hydrologic and geomorphic aspects of riparian forest ecology on the lower San Miguel River, Colorado

The broad goal of this research was to elucidate riparian forest dynamics in a hydrologic and geomorphic context. Using the San Miguel River, CO as a case study I assessed the history and nature of the streamflow regime, determined the nature of geomorphic adjustment to that regime, and compared patterns of forest establishment with episodes of bottomland disturbance.; The lower San Miguel River has a mixed population of flood-types, including spring plateau snowmelt, summer alpine snowmelt, and late summer rainstorms associated with the Southwest Monsoon. Based on streamflow records from Uravan, CO snowmelt floods average 110 m{dollar}sp3{dollar}/s and account for 60% of all recorded annual peak discharges. A regionalized estimate of flood frequency indicates that snowmelt floods are not likely to exceed 2-3 times the mean annual flood (Q{dollar}rmsb{lcub}maf{rcub}),{dollar} even at a 100 year return interval. Monsoonal rainfall floods average 105 m{dollar}sp3{dollar}/s and account for 40% of annual peak discharges, but may be as large as 4-5Q{dollar}rmsb{lcub}maf{rcub}{dollar} at the 100 year return interval. Mean daily snowmelt floods periodically equal or exceed Q{dollar}rmsb{lcub}maf{rcub}{dollar} for several days, while even the largest monsoon season flood (252 m{dollar}sp3{dollar}/s) did not result in a mean daily discharge that was equal to Q{dollar}rmsb{lcub}maf{rcub}.{dollar}; Using hydraulic geometry measurements from 51 cross-sections I estimated that initiation of sediment movement corresponds with a discharge of 42 m{dollar}sp3{dollar}/s, and occurs about 19 days/year. Widespread reworking of channel deposits, however, only occurs under sustained bankfull or larger flow conditions.; I used tree ring counts to determine that a major stand establishment episode occurred in 1985-86, after snowmelt flooding in 1983-84 delivered 14 days of mean discharge greater than Q{dollar}rmsb{lcub}maf{rcub}.{dollar} The specific ages of another cohort were less conclusive, but are probably associated with snowmelt flooding between 1941 and 1944, when 17 days had mean Q greater than bankfull. A small cohort from the early 1970's established on top of fine-grained flood deposits, suggesting that monsoon flooding may play a unique but lesser role in creating establishment sites. Aerial photography analysis showed that channel pattern changes have been relatively widespread over the past 50 years. These changes are primarily caused by post-flood channel narrowing and channel wandering. The expansion of bare alluvium through time was best explained by correlating unvegetated areas with periods of sustained flooding equal to or exceeding Q{dollar}rmsb{lcub}bkf{rcub}.{dollar} Lesser, more localized channel changes may also occur in response to short pulses of sediment transport deposited after a monsoonal runoff event.; Patterns and mechanisms of fine sediment deposition were examined to determine the history of a discontinuous mantle through the study area, and to test the hypothesis that these deposits are accreted episodically rather than gradually. Results suggest that the mantle was deposited by one or two monsoon season flood events that were heavily laden with fine sediments from Dry Creek Basin, a tributary of the lower San Miguel River.