3 of 4: Putah Creek's Floods and Dam
A large-scale restoration project in Putah Creek is scheduled to begin in August 2011. The project will narrow the channel, and create floodplains and walking paths. This is the third in a four-part series intended to inform those interested in the project.
Putah Creek was once a very flood-prone system, with storm flows reaching almost 375,000 gallons per second (50,000 cfs) once every five years. With Monticello Dam, current peak storm flows rarely exceed 74,500 gallons per second (10,000 cfs) because 90% of the watershed is upstream of Monticello Dam.
Water is not the only thing retained behind the dam. The dam also traps gravels and cobble, leaving the creek with little ability to replenish its supply. Gravel and cobble are critical for insects, the base of the food chain, and as fish spawning habitat.
The completion of Monticello and Putah Diversion dams severely limit the creek’s ability to move gravel and cobble downstream. However, gravel mining continued in some areas after dam construction. The pools created by mining were are longer filled by natural processes.
The Putah Creek streambed and channel have been straightened by a number of groups.Settlers needed a way to keep Davis and surrounding farm lands from flooding, so they constructed a wide, straight channel south of Davis and moved the creek into it.
Putah Creek used to flow near downtown Davis in what is now the UC Davis Arboretum channel, but early settlers began the process of moving it south of town to its current channel in 1871, a project finished off with levees by the Army Corps of Engineers in the late 1940s.
Straightening the channel for flood conveyance also occurred at various locations upstream.
Straightening the channel alters the way the water’s energy impacts the channel. For comparison, when we climb a hill in our car we travel what are known as switchbacks: the curves in the road which make our journey a little longer, but make the road between the top and bottom less steep and ensure a controlled descent.
Taking the curves out of Putah Creek was like removing a road’s switchbacks—it made essentially made the creek steeper as it made its way to the Sacramento River.
This slightly steeper channel caused water to flow faster and with greater force. The base of the Railroad Bridge in Winters reveals three feet of rough-poured concrete which was originally below ground. This indicates the channel has dug itself down three feet since 1906 when the bridge was built, caused in large part because of channel straightening downstream.
Putah Creekhas also had a lot of vegetation removal.Until the 1950s, the Army Corps of Engineers removed most vegetation from Putah Creek’s banks to enhance flood capacity, not realizing the impact this would have on erosion to the streambed. In addition to increased erosion, the bare banks became a haven for introduced species, such as arundo, eucalyptus, and tree of heaven.
With the removal of native vegetation, invasive weeds were able to establish along the creek. Himalayan blackberry and arundo (false bamboo) trap sediment in high flows, and form hard areas which deflect, or push back, storm flows which hit them. They don’t react to storm flows the same way native vegetation does.
Unlike our native streamside vegetation loses its leaves in the winter and is flexible enough to bow over during high flow, invasive weeds such as arundo are rigid. This can both trap sediment, and cause erosion by diverting the flow of water onto the bank.
Evergreen streamside weeds are particularly problematic. Leaves increase flow resistance, slowing floodwaters and robbing water of the energy necessary to transport fine sediment downstream. As sediment accumulates at the base of blackberries and arundo, the floodplains grow higher above the creek, making it hard for native seedlings whose roots seek water before the summer heat returns.
The major restoration project scheduled to begin in August 2011 will remove invasive plants, grade the floodplane, a create a meandering course for Putah Creek between the Winters Car Bridge and Interstate 505.
The Creek’s banks will be planted with native trees and plants, and walking paths will give residents and visitors the opportunity to walk on both the north and south banks along this wonderful stretch of public land.