River-Lab

In recent years, river restoration has shifted towards natural, process-based approaches rather than fixed, engineered solutions. This new emphasis on natural approaches was implemented in a restoration design and subsequent experiment in the Tilden Park Golf Course reach of Wildcat Creek in Berkeley, California, USA. The restoration design implemented step-pools to address bank erosion and encourage channel stabilization, while the experiment highlighted the creek’s ability to self-organize these desired step-pool formations without the need for human interference. The Restoration and Experimental Reaches were monitored consistently from 2012 to 2017 and again in 2022, and found that after the addition of step-pools, the channel bed remained fairly stable without significant aggradation or erosion. Our study conducted a post-project monitoring of the Restoration and Experimental Reach, to gain further insights on potential changes in the channel and step-pool morphology, as well as identifying three potential contributing factors: vegetation growth, precipitation, and disturbances.

Previous monitoring in 2022 noted that vegetation management greatly influences channel morphology, and willow growth in the Restoration Reach could thereby negatively impact channel stability. Further, since the most recent monitoring in 2022, a series of large winter precipitation events may have altered the channel morphology but have not yet been accounted for in analysis. Additionally, a willow fell into the Experimental Reach during the winter of 2015-2016, accumulating debris and causing aggradation. While the effects of this disturbance are noted through cross-sections taken in 2016 and 2017, it is unclear how the disturbance impacted step-pool morphology as well as longer-term trends in channel morphology. Through three days of field work, we surveyed cross sections, performed pebble counts, and observed the step-pool formations and coinciding slope in the restoration and Experimental Reach. Informally, we also replicated photos taken yearly at various perspectives along the creek and recorded occurrences of riparian species. We situated our data in the context provided by documents from and interviews with Dr. Anne Chin and Dr. Patina K Mendez, as well as Restoration Design Group (RDG). Results show that channel morphology, sediment composition, and step-pool morphology are heavily influenced by vegetative growth, precipitation, and the log jam disturbance. Specifically, the channel cross sections aggraded due to willow growth and the log jam, though the Experimental Reach impacted by the log jam has recently shown signs of incising and returning to pre-log jam conditions. High flows greatly impacted sediment composition, as high flows allowed for increased transport capacity, flushing out the coarser material added by RDG in the 2012 restoration. Without replenishment of coarse material, the sediment supply returned to pre-construction fines, decreasing grain size. High flows and the log jam also influenced step-pool morphology, high flows allowed for an increase in the number of step-pools, and the log jam allowed for a decrease in average slope and average five largest rock sizes.