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Vegetation in the Floodway

"Native vegetation structure is quantifiable and its response to flows is predictable."

The river floodplains of the Central Valley are ideal for plant growth. The soils are the richest in California (composed of deep sand and silt loams) and the soil water table is within reach of plant root systems. Along with the Mediterranean climate, the soils and hydrology of the Central Valley floodplains create the conditions for rapid plant growth.

Riparian Sanctuary

The rich soils, accessible water table, and mild Mediterranean climate in the Central Valley create excellent growing conidtions for native plants. This ideal setting for plant growth also supports a productive agricultural industry.

The native trees and shrubs are adapted to the natural hydroperiod of the river and are aggressive at establishing from seed, if there are no dams on the river. With dams present, a different hydroperiod is established that typically does not allow seedling establishment of the native willows and cottonwoods, or flows are reduced such that periodic scouring of point bars and the floodplain rarely or never occurs, leading to encroachment and narrowing of the active channel. Under dammed conditions many invasive plant species are able to colonize the floodplain, rendering the vegetation a lower habitat quality for wildlife.

The floodways of the Central Valley consist of the Sacramento, Feather, and San Joaquin Rivers (and several of their tributaries) that have had large levees (30-40 feet high) built along both sides of the rivers. The levees are aligned to carry a specific design maximum flow in excess of 300,000 cubic feet per second (cfs) on the Feather River, more than 600,000 cfs on the mid and lower Sacramento River, and about 50,000 cfs on the San Joaquin River.

Sac. River Flood Control System

The flood control features along the Sacramento River are extensive. The map above shows the general flood control features along the Sacrmento River, last updated in November of 2003. This map is used with persission from DWR; this and other maps can be found at http://www.water.ca.gov/floodmgmt/pubs/# .

When the levee system was designed over one hundred years ago, the rivers of the central Valley were filled with sediment as a result of hydaulic mining for gold in the Sierra Nevada. Hydraulic mining involves the use of high pressure water focused at hillsides to wash away the soil and lose rock to expose gold. The soil and rock filled the canyons and the mountain streams carried it down into the Central Valley where it buried the floodplains in Central Valley. At one point in time, the bed of the Sacramento River was higher than the waterfront streets in the sity of Sacramento (similar to Delta Islands today).

The first government-sponsored effort to provide flood control to the Central Valley started in the early 1900s. The levee design engineers intentionally placed the levees relatively close to each other so that flows would be deep, for the purpose of mobilizing the mining sediments, and to flush them into the Delta and out of the river channel and the new floodway. The result has been extremely unnatural depths of flood flows - in excess of 14 feet deep over the floodplains along the Feather River! These depths were never achieved before the levee system was constructed. Flows of this depth exert a tremendous shear-force (weight) upon the bed, floodplain, levees, and vegetation. Today, much of the mining sediment has been flushed from the sytem, so the deep, fast-moving flood waters are now eroding the levees, hence the levee-repair program that is currently under way.

River Channels

Channel cross section in 1900 during extensive mining, and in 2000 after the consturtion of levees.

Vegetation is perceived by most people to be all the same at slowing flows, whether it is composed of "trees or brush," or any combination. Vegetation in the floodway is perceived to be a problem for 3 reasons:

  1. Vegetation will "plug-up" the sytem because it will grow large and slow the velocity of flows, causing floodwater to back-up, or erode a hole or gash in the levee.
  2. Vegetation will direct high velocity flows into a levee or other flood control structure causing erosion or some other compromise (stack-up at a bridge).
  3. Vegetation will cause the elevation of the flood flows to be higher solely because its mass will displace water causing the flood elevation to over-top the levee.

The following web pages will explain why these common perceptions do not reflect the true behavior of riparian vegetation in the floodways that is seen in flume experiments and hydraulic modeling.

The Flume Study

Willows in the Flume

River flows can be powerful, and because of the independent movement of individual water molecules, flows can generate high turbulence. In a recent flume study at the J. Amorocho Hydraulics Laboratory at UC Davis Large Flume, measured depths and velocities of flows were tested on four species of flexible stem riparian plants and on bare soil to quantify the effects of vegetation within the floodways.

Hydraulic Modeling and Restoration Design

Hydraulic Model

O'Connor Lakes is a River Partners' restoration project that had to be designed to have no impact on floodway flows. Using hydraulic modeling to understand the variation in flow velocity across the floodplain, riparian vegation was planted that remained flood neutral and provided wildlife habitat.

Recommendations for Floodway Vegetation Managment

Plant Design

Restoration designs benefit from studies of vegetation response to flows and from modeling the hydraulic characteristics of the project site. The restoration planner can modify the density and arrangement of riparian plants to produce a flood neutral design.