Within the tributaries that drain to San Francisco Bay, there exists a transition between fluvial and tidal processes and conditions. The upstream boundary of this transition, called the head of tide (HoT) zone, can be defined as the inland limit of the effects of average high tides on tributary flows and water surface elevation. This zone is characterized by unique and diverse assemblages of plants and animals, cultural resources, as well as a vulnerability to out-of-channel flooding during high river flow and high tide conditions. As many Bay Area municipalities are built near the HoT zone, there is a growing concern about managing the flooding risk as well as the aquatic resources in the HoT zone for current conditions and future conditions when rapid sea level rise causes the HoT zone to migrate inland. The first step in developing effective management strategies needs to be creating a process, or protocol, for determining where the HoT zone is now and where it will likely be in the future.
SFEI completed a pilot study focused on creating a framework for a rapid protocol that can be used to delineate the current and future HoT zone for San Francisco Bay tributaries using both “desktop” and field investigations. The protocol was developed by examining data collected at six tributaries that represented a broad range in watershed size and channel gradient. The desktop investigation used publically available spatial tools as a “first cut,” coarse estimate of the current HoT zone location. The field investigation involved examining multiple physical and biological indicators of both the current and future HoT zones and is intended to refine the estimate given by the desktop investigation. The data were then analyzed to determine the indicators that are most effective at rapidly identifying the HoT zone location and extent.
The study found that a combination of desktop and field investigations can be used to develop rapid yet reasonable estimates of the current and future HoT zones for the San Francisco Bay tributary sites examined. These findings are encouraging and suggest that a robust, validated protocol appropriate for Baywide application can be developed with data from more representative Bay tributaries. SFEI plans to continue protocol development in close coordination with regional management agency partners.
This project was completed in June 2014