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The impact of human activity on deltaic sedimentation, marshes of the Fraser River Delta, British Columbia Hales, Wendy J.

Abstract

To determine the impact of human activity on marsh sedimentation rates and growth patterns, the recent history of marsh development at the mouth of the Fraser River delta has been constructed. Lateral marsh growth is assessed through aerial photographs, while vertical sedimentation rates are resolved using sediment cores. Changes in marsh sedimentation rates and patterns were compared with historical information about natural events and human activity in the study area, to show that marsh sedimentary sequences record the impact of human activity. A total of 35 cores were collected, of which the seven longest (3.03 to 2.75 m in ground penetration length) with the best lamination sequences were analysed for 1 3 7Cs, heavy metal concentration, organic content, and sediment density and texture. The combination of these analyses enabled the cores to be dated to 1894, prior to large scale human interference in the deltaic system. To enable correlation between the cores, they were analysed for environmental magnetism. Aerial photographic mosaics were created for the years 1930, 1954, 1974 and 1994 in order to determine the rate of lateral marsh growth. Historical maps were also examined, but due to irresolvable map error, it was not possible to determine marsh areas accurately prior to the advent of aerial photography. The sedimentary record indicates that the marshes experienced rapid aggradation, an average of 2.10 g/cm /a, between 1910 and 1954, the period during which major river training structures were constructed in the study area. Laterally, the marshes also grew rapidly during this period: between 1930 and 1954 they experienced a 16 % increase in area (93 x 103 m2/a), including losses due to land reclamation. After this period, marsh aggradation slowed, as did lateral growth in most areas. The decline in vertical growth is attributed to the marshes attaining a balance with the new environmental conditions created by river management. This research shows that sedimentary sequences from undisturbed estuarine marshes provide historical information about the impact of human activity on sedimentation rates and patterns. In addition, earlier portions of the marsh sequences can be used to establish information about natural environmental conditions for comparison purposes in areas where such background data are otherwise unavailable.

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