The Physical Characteristics and Human Impacts that Affect the Sacramento River’s Morphology
...Straight channels and meander channels are different because the flow velocities and velocity gradients are located in different parts of the channel. In straight channels, the highest flow velocities and highest velocity gradients are located near the center of the channel, with the lowest gradients occurring near the banks (Mount, 2003). In meander channels, the velocities and gradients are always located adjacent to the steep-walled cut banks (Mount, 2003). Furthermore, straight channels and meander channels also differ because the concentrations of bed shear stress are located at different areas of the channel, thus areas of erosion in the two channels are different as well. The Straight channels tend to deepen because the concentrations of bed shear stress are at the center of the channel (Mount, 2003). Meander channels tend to expand laterally due to the eroding of the channel walls by the concentrations of bed shear stress along the cut banks (Mount, 2003). Straight channels and meander channels also differ at where sediment is deposited. In Straight channels, sediment is deposited in the channel bars or along the margins of the channel, where as in meander channels, sediment is deposited on the inside of the channel bends (State of California. The Resources Agency. 2003). The most common channel pattern of the Sacramento River is the meandering pattern (State of California. The Resources Agency. 2003). The section of the Sacramento Valley, where the Sacramento River meanders, is in the Red Bluff-Chico Landing and Chico Landing-Colusa Reaches, and portions of the Keswick-Red Bluff Reach. The Sacramento River meanders by migrating back and forth across its flood plain. The balance between erosion on one side of the channel and deposition on the opposite side is the driving force behind the meandering of the Sacramento River (fig.3) The portion of the valley where the Sacramento River migrates is called the meander belt (fig.4). Over time, the Sacramento River moves laterally due to the magnitude, duration, frequency, and velocity of the river’s flow (Morisawa, 1968). This leads the Sacramento River to develop a meander belt within its floodplain because the river is adjusting to balance erosion with deposition. Characteristics of the Sacramento River’s meander belt are the width of the corridor, the channel sinuosity, and the rate of channel movement. The exact measurement of the Sacramento River’s meander belt is difficult to determine because of human activities such as building of levees has prevented the river from meandering at certain locations. However, an approximation of the distance of the meander belt can be determined by measuring the width of the river’s floodplain. Human Impacts that Altered the River’s Morphology In the last 60 years, the Sacramento River’s flow regime has been affected by major man-made structures such as dams. Dams such as the Shasta Dam on the upper Sacramento River, Oroville Dam on the Feather River, and Folsom Dam on the American River are the largest dams in California. These dams are an example of major man made structures built on the rivers in the Sacramento watershed. These large dams have altered the Sacramento River’s geomorphic process of sediment transport. The amount of sediment that is transported by the Sacramento River is important because the processes of sediment supply and sediment transport are fundamental drivers of channel and floodplain morphology (FERC, 2002). These dams affect the sediment transport in the Sacramento River because they intercept and trap sediment in their reservoirs. This severely impairs the basic sediment transport processes of the Sacramento River. Interception and trapping of sediment by dams reduces the normal amount of sediment entering the Sacramento River and its tributaries by decreasing the water flow velocity and inducing deposition. This cause changes in the Sacramento River’s flow regime because it cuts off the supply of sediment that normally would be deposited on to the valley floor or delivered to the watershed outlets. Trapping of sediment also causes the rivers to erode channels downstream. This is because the supply of water released from these dams is relatively clear and contains little or no sediment (The Natural Heritage Institute. 2001). Therefore, the river erodes the channels in an attempt to gain back the sediment that it has lost. As a result, trapping of sediment by dams alters the Sacramento River’s physical attributes and functions such as channel shape, channel stability, channel movement, and channel capacity. In addition to the trapping of sediment, these dams that are constructed on the Sacramento River and its tributaries also changes the duration, frequency, and magnitude of peak flows. This alters the Sacramento River’s flow regime because it changes the natural flow process of the Sacramento River. These dams store peak flows in their reservoir for flood control, irrigation and water supply. Since the majority of sediment transport occurs during high flows, reducing the magnitude of these flows reduces the total sediment transport capacity down steam and changes the amount of sediment deposition. Changing the duration, frequency, and magnitude of peak flows also reduces bank erosion rates. From 1946 to 1980, the Sacramento River has experienced a 25% reduction in bank erosion rates (Kelly, E., 2000). Bank erosion is also related to the migration of a meandering stream channel (Kelly, E., 2000). Since the erosion rate of the Sacramento River has been reduced, it is likely that its meander pattern has been altered as well. Reductions in sediment transport capacity and bank erosion rates coupled with changes in the amount of sediment deposition and meander patterns, supports that dams on the Sacramento River and on its tributaries affect the natural flow process of the Sacramento River. Along with the construction of dams, the hydrology of the Sacramento River has also been affected by historical hydraulic mining. Hydraulic mining is a process which involves spraying large amounts of water against canyon walls and separating the generated runoff in order to obtain gold (Alpers, C and Hunerlach, M., 2000). Hydraulic mining started in California around 1850 and operated on a large scale until the late 1880’s (fig. 5). More than 1.5 billion cubic yards of gravel in the northern Sierra Nevada Mountains were worked (Alpers, C and Hunerlach, M., 2000). This left a lasting effect on the Sacramento River watershed because this type of mining stripped the ground of soil, sand, and gravel above bedrock. Hydraulic mining operations were responsible for dumping large amounts of sediment into the rivers of the Sierra Nevada Mountains. This choked up many of the tributaries, decreasing the amount of water being discharged into the Sacramento River. It is estimated that hydraulic mining has added 1.5 billion cubic feet of sediment to the Central Sierra Nevada rivers (Alpers, C and Hunerlach, M., 2000). This large accumulation of sediment put into the rivers also worsened naturally occurring floods. River flows were increased in size and strength due to the abundance of sediment put into the rivers. This resulted in severe channel incision and bank...