Beehive Bend Subreach Addendum to: A Pilot Investigation of Cottonwood Recruitment On The Sacramento River

• Based on analysis of hydrologic data, dendrochronologic data, historic channel mapping, and aerial photography riparian recruitment appears to occur approximately once per decade in the post-regulation period. A comparison to stands that recruited in the pre-regulation period is needed to quantify differences between pre and post regulation period temporal and spatial characteristics of riparian forest regeneration. • Recruitment did result from the recent large flood events of 1983-1986, and 1995-1997. Although this recruitment follows a one in ten year frequency, recruitment events may now be limited to larger, less frequent events. • Similar to the previous study, bank protection appears to further limit recruitment, although the specific degree and mechanism are difficult to quantify. Recent recruitment was limited at the previous study site, which has bank protection and a levee in close proximity. Although large in frequent floods may be required to produce it, recent recruitment did occur at the new study sites. These study sites do not have bank protection or levees in close proximity. • Recent recruitment appears to favor the downstream ends of point bars and these locations may best meet box model criteria. A complimentary study found that much of this recruitment is dominated by willow species as opposed to cottonwoods. It is unknown if this is a result of altered hydrology on the Sacramento River. • If large, infrequent floods do not occur to produce a forest cohort, a refined recruitment box model is available to guide a recruitment flow for the Sacramento River. The range of appropriate elevation for recruitment is now 3 to 6 feet above mean low water (8,500 cfs). This new information results in significantly less water required to implement a managed recruitment flow than identified in a previous study. • A decreasing elevation of recruitment through time may indicate channel degradation, as do other information sources. • A number of flow management strategies were evaluated, which require a range of water for implementation. When compared to actual wet year flows at these study sites, these managed strategies can require up to 37% less water. • A managed flow strategy that takes advantage of natural conditions that narrowly miss recruitment needs may best meet multiple water use objectives. Spring 2003 conditions narrowly missed recruitment needs. Based on the information from this study, a change in the timing and recession rate of spring 2003 conditions could have formed a recruitment flow. …