A Tidal Hydrology Assessment for Reconnecting Spring Branch Creek to Suisun Marsh, Solano County CA: Predicting the Impact to the Federally Listed Plant Soft Bird's Beak

ABSTRACTSpring Branch Creek drains a 2,670-acre watershed into tidally influencedSuisun Marsh in Suisun City, Solano County, CA. A farm levee road and berm that were constructed in the 1930s to drain the site for agriculture created an abrupt transition between fluvialand tidal systems. In the 1990s, the landowner Solano Land Trust installed two four-foot culverts beneath the levee road in attempt to partially restore the exchange of brackish tidal water with fresh water. Ten years later (in 2000), a population of federally listed plant soft bird’s beak (Chloropyron molle ssp. molle, syn., Cordylanthus mollis ssp. mollis) was reintroduced in the high marsh zone under these altered hydro-logical conditions and is now a thriving population of 100,000 individuals. Now, a proposal to remove the levee completely, and reconnect fluvialand tidal systems, raised concern that the livelihood of this population might be compromised by altering the hydrological conditions. I conducted a tidal inundation analysis to describe the differences in current inundation frequency, duration, and depth in the high and low marsh zones, and above and below the Spring Branch Creek culverts. I also created a water surface model to predict how these hydrological differences will change following reconnection. Results show that hydrological conditions in the high marsh zone, where soft bird’s beak occurs, will not significantly change following reconnection, with tidal changes of only 5-6 cm. Water elevation ranges in the low marsh zone, however, are predicted to decrease as much as 55 cm, and could possibly affect low marsh vegetation. Threats beyond the proposed hydrological reconnection that directly impact the plant include competition from non-native species. Thus, monitoring of population viability should continue after reconnection.