Cooper D. Stacey, Geological Survey of Canada, Natural Resources Canada, Sidney, Canada
Philip R. Hill, Geological Survey of Canada, Natural Resources Canada, Sidney, Canada
D. Gwyn Lintern, Geological Survey of Canada, Natural Resources Canada, Sidney, Canada
Evolution of a recent channel-levee complex inferred from levee deposits on the Fraser Delta front, British Columbia, Canada
The Fraser Delta hosts a population of over 500,000 including the municipalities of Richmond and Delta. The main arm of the Fraser River has been fixed in place by construction of a jetty focusing sediment deposition at the mouth of the main distributary channel. There is a history of submarine slide events at the delta crest which pose substantial risk to coastal infrastructure near the delta front. Natural Resources Canada has a project aimed at determining the causes and risks associated with these slides. A submarine channel-levee complex extends seaward from the main distributary channel.
Sediment cores from the levee deposits indicate that sedimentation is characterized by sandy mud, interpreted as continuous deposition by river plume suspension fall-out, and two distinct kinds of sand beds which represent two different processes. The first type of sand bed (facies 6) is interpreted as the deposit of channelized turbidity currents from slides involving large volumes of material at the upper reaches of the tributary channels. The second type of sand bed (facies 5) is interpreted as a deposit from river generated turbidity currents. Facies 6 beds are restricted to the channelized margin of the delta front and the upper delta in areas where gully formation is prevalent. Facies 5 beds are distributed across the active delta front and likely represent deposition from sheet-like flows. Ages of sand beds were resolved by establishing a sediment accumulation rate using excess 210Pb activity from mud intervals inferred to represent constant deposition from the Fraser River.
Facies 6 sand beds often occur in sets of 2 to 4 beds and individual bed sets correlate to known large slide events with a return interval of 10 to 15 years during the past 40 years. Deposits of smaller events, including Facies 5 beds, occur on average every four to five years. Event ages are compared to large spring floods from the Fraser River and seismic activity to determine any causal relationship. It is concluded that there are likely a combination of other factors which determine the volume of slope failure including cumulative over-steepening and increased pore pressure.
Levee sedimentation is linked to channel-levee construction and three phases of evolution, characterized by a general fining up, are identified. The first phase corresponds to the installation of the Steveston Jetty from 1912-1932 and the establishment of a new channel position. During this phase channel relief was low and overspill deposits were frequent resulting in very thick sand deposits on the levees. During the second phase of levee growth channel relief increased and overspill events were less common resulting in deposits that are not as thick. During the third and current stage, channel relief has increased to a point where channel overspill events are far less common resulting in levee deposits that are characterized by thick mud sequences with thinner, less frequent sand beds.
Special Session 1: Sediment Transport monitoring in submarine canyons
submarine slide, turbidity current, delta, channel-levee complex, levee deposit, channel evolution