The evolution of the Sacramento River during the Late Quaternary and the influence of an “inherited” topography on modern floodplain sedimentation rates
Thesis or dissertation
University of Exeter
Reason for embargo
Standard 18 month embargo for publications
This study investigates the evolution of the Sacramento River since the late Quaternary, and the influence of this “inherited” topography on last century’s floodplain deposition by analysing and dating fluvial deposits at one of the key sites in the Sacramento River flood network. This thesis investigates older deposits and also modern (last-century) depositional history as means of interpretation. Take home message 1: This study provides an extensive time constraint on Late Quaternary fluvial sediments in the Western US. It indicates a link between Late Quaternary climate instability and fluvial deposition and erosion in the research area. The main periods of deposition are around 77 ± 25 ka, between 33 and 22 ka, and after 6 ka, with a period of surface stability on indicated by the development of a soil between 22 and 6 ka Llano Seco Terrace. The floodplain channels are of Holocene age and origin, potentially the result of an incomplete avulsion.During the period around ~ 75 ¬± 25 ka, the progradation of the Stony Creek fan system towards the centre of the valley (represented by vast gravel deposits of Coast Range origin), relocated the Sacramento River to the ~ 4 – 6 km east of its current position. Between ~ 33 ka and ~ 22 ka, shallow (1 – 2 m) fluvial sands were deposited covering large parts of the floodplain. After 22 ka soil development suggests a stable surface in the eastern part of the research area. This indicates the existence of an incised Sacramento River around the Last Glacial Maximum in the vicinity of the recent channel. In the early Holocene a meandering system is established, covering the soil of the Llano Seco terrace with fine-grained overbank deposits. Different meander generations have been dated, because a westward movement of the Sacramento River has been observed. The oldest (easternmost) dated meander dates to 2.4 ± 0.1 cal PB. At least 2 older generations of meanders are topographically visible that have not been dated. The prominent ephemeral floodplain channel systems in the research area have developed almost exclusively in sediments of Holocene age and origin. They are therefore interpreted as the product of a Holocene avulsion that remains incomplete, because the Sacramento River is not able to incise in the underlying gravel deposits. Take home message 2: XS 210Pb profile analysis used tool to investigate the influence of sedimentation and land use on XS 210Pb activity profiles, withthe CIRCAUS/CNAXS dating technique further enhanced for anthropogenically influenced landscapes. Take home message 3: Using high-resolution XS 210Pb analysis it was possible to reconstruct sedimentation rates, sometimes frequency of deposition, as well as identify the influence of land-use on XS 210Pb profile development. Results showed that the floodplain channels along the Sacramento River are an excellent conveyor of sediments and are able to disperse ~ 33 % of the average annual suspended sediment load over longer timeframes (50% of each biannual flood) to the distal parts of the floodplain. The main controlling factor for sediment deposition in the research area is the distance from the main river channel. But a total of 60 % of the suspended sediment load is transported more than 500 m away from the main stem Sacramento River and removed from areas that are frequently reworked.
Jones, Richard Thomas
PhD in Geography