Sedimentary Budget - Sediment Compartments and Littoral Cells - Sources - Rivers

Rivers

Rivers are major point sources of sediment contribution to the coastal sedimentary budget, this being particularly true of coasts with a steep gradient, where rivers directly dump their sediments at the coast. Coasts with a low gradient may lose river sediment to estuaries. Sediment delivery to the shoreline can be very intermittent mostly occurring during floods with the increase in flow typically creating an increase in sediment supplied to the coast. Some rivers are referred to as ‘large’ because they produce high amounts of sediment for which to nourish the coastal environment. For example, the Waimakariri River on the coast of Canterbury, New Zealand produces 77% of sediment supplied to the Pegasus Bay coastline. This is not always the case with some rivers referred to as ‘small’ because they struggle to supply enough sediment to keep there coastlines from eroding, for example, the Rakaia River South of Banks Peninsula in Canterbury, New Zealand.

The construction of river dams for flood control and hydropower reduces sediment supply to many coastlines due to the trapping of sediment and the decrease in flood peaks and flood intensity. In places such as Southern California of the United States, in particular the San Luis Rey River, dams are built to control flooding of properties along the river. Ironically, in doing so, this is contributing to the damage of coastal properties due to a lack of sediment produced to protect the beaches. Another example is the Aswan Dam constructed on the Nile River, Egypt in 1964. Prior to the construction of the Aswan Dam, the Nile River delivered 60-180 million tones of sediment and water to the Mediterranean Sea every year. Sediment supply is now almost zero which has produced a significant imbalance to the near shore sedimentary budget, creating major erosion and shifting of sediment along the coast.

The effects of sediment trapping due to dams can be exacerbated when combined with other activities such as in-stream gravel mining. Excavation of gravel from a river bed forms pits within the channel profile which can trap much of the incoming bed load sediment, preventing or slowing it from reaching the coastline. Mining can also reducing the amount of overall sediment available for transport, especially when it occurs down stream from dams. For example; approximately 300,000m3 of gravel is extracted from the San Luis Rey River in South California every year, which is almost 50 times more than the bedload sediment yield after the dam was built. Thus, removing more bedload sediment further decreases the sediment yield available to the coast.

The removal of natural vegetation for cultivation and land use can increase soil erosion resulting in an increase in sediment yield transported by rivers to the coast. For example; in Westland New Zealand this has had a cumulative effect with clear felling of trees increasing in river sediment yield up to eight times.