| Abstract/Description or Keywords |
Enhanced sediment yield associated with forestry activity is well documented. While some studies have focused on assessing the increase in sediment concentrations of streams, the extent to which sediment is transmitted down valley to storage areas such as lakes remains to be established. There are also unanswered questions about long-term trends in sediment yield. It has been suggested that the study of lake sediments may provide a means to monitor the effects of forestry-related activities. However, a better understanding of the connection between catchment disturbance and lake sedimentation is required to assess the suitability of this approach. To explore these questions, lacustrine sedimentary records from three logged basins and one unlogged basin on the west coast of Vancouver Island, British Columbia were analyzed for physical and chemical properties. Core correlations were based on x-radiography and trends in organic content and magnetic susceptibility. Chronological control provided by 210Pb and 137Cs activity demonstrated that 10-30 cm cores record 100-150 years of sediment deposition and allowed the calculation of sediment accumulation rates. Historical information about both natural and human disturbance in the study areas was compared with changes in sediment characteristics and sedimentation rates. Trends in sediment yield and indicator properties associated with disturbance were thereby identified. The results indicate that increases in sediment yield coincide with forestry-related disturbances, natural disturbances such as rainstorm events, and other human activities such as mining. The identification of the sedimentary signature of forestry-related activity is confounded in one of the logged basins by other catchment disturbances. Depositional events identified on the basis of x-ray stratigraphy and sediment properties also coincide with historically documented instances of localized catchment events such as a landslide or forestry-related mass movement in gullies. Of all the sediment properties, changes in the relative proportions of organic and inorganic sediment fractions are most sensitive to upstream catchment conditions as evident from the correspondence between the general change in sediment composition and disturbance history. Methodologically, the results of the study demonstrated that the lake sediment approach may be successful in monitoring the effects of forestry-related activities, although this largely depends on the precision of chronological control, a rigorous sub-sampling strategy and the use of multiple cores. Limitations of the sediment chronology were investigated and show that underestimates of sediment yield may result from assumptions made in the modelling of chronological data. Not all lakes may be suited to this technique due to the resolution required and given that other disturbances occurring in the catchment may obscure inference from the sediment record. |
