Presentation on theme: "Introduction Western Oregon is home to a very productive temperate forest and with this valuable economic resource comes the concern that logging activities."— Presentation transcript:
Introduction Western Oregon is home to a very productive temperate forest and with this valuable economic resource comes the concern that logging activities may increase sediment erosion and consequently impact the water quality, channel stability, and riparian ecosystems. In the 1950s, a series of watershed experiments were initiated in the H.J. Andrews Experimental Forest. This 30 year examination of hydrologic, geomorphic, and biological effects of timber harvest provided lengthy data on the sediment transfer in three separate wooded locations. Another study explored the 50 year record of sediment deposits behind the reservoir at Dorena Lake in western Oregon providing some insight into the affects timber harvesting has on the discharge rates of watersheds. Dorena Lake Study The 686 sq km Dorena Lake watershed receives an average of mm of rainfall per year. The dominant land use for this area is forestry because the primary vegetation is Douglas fir, western hemlock, and western red cedar. Dorena Lake was constructed in 1949 by the United States Army Corps of Engineers as a part of the Willamette Basin flood-control project. Investigation into the influence flood events and logging have upon sediment yield was launched over a 50 year period (Ambers 2001). This study used sedimentation surveys, sediment cores, and stratigraphic hydrologic records to learn more about the sources of sediment build up behind the reservoir. Eight sediment cores were collected from the thick sediment areas of Dorena Lake and subjected to intense study (Figure 1). Each core represents a complete thickness of lake sediment compiled over dozens of years. The last 35 years of reservoir operation produced an intermediate sedimentation rate of 2.0 cm/year. Particle analyses from the first three samples revealed that the majority (54%) of the sediment in the cores was silt with lesser amounts of sand and clay. Specific storms such as the Christmas flood of 1964 were also distinctive across much of the lake bottom (Ambers 2001). References Cited Ambers, R.K., 2001, Using the sediment record in a western Oregon flood- control reservoir to assess the influence of storm history and logging on sediment yield: Journal of Hydrology, v. 244, p Grant, G.E., and Wolff, A.L., 1991, Long-term patterns of sediment transport after timber harvest, western Cascades Mountains, Oregon, USA, in Peters, N.E., and Walling, D.E., eds., Sediment and stream water quality in a changing environment: Trends and explanation: Proceedings of the Vienna IAHS Symposium, Vienna, Austria, August, 1991, International Association of Hydrological Sciences Publication 203, p The Influence of Timber Harvest on Sediment Transport in Headwaters of the Willamette Basin Prepared by: Levi Hogan Table 1. Characteristics of three experimental watersheds (Grant and Wolff 1991). Figure 2: Annual Sediment yields for Watersheds (Grant 35). Experimental Forest Study For this study, three small adjoining watersheds were selected because of their similar sizes, aspects, and topography (Table 1). Watershed 1 was harvested from using a skyline suspension system which minimized surface soil disturbance. Watershed 2 served as a forested control for the experiment. The third watershed contained 2.64 km of roads and three discontinuous clear cuts logged in the winter of (Grant and Wolff 1991). Discharge from these three watersheds was monitored since 1953 along calibrated flumes at the downstream end of each watershed. Vertical sediment grab samples were also taken from the head end of each flume during and between storms. Sampling of the suspended and bed load sediment at the flumes was recorded through 1988 (Grant and Wolff 1991). However, since suspended sediment discharges were not continuously measured annual sediment yield was calculated using empirical models that related sediment flux to hydrograph characteristics. Separate multiple regression models were developed for rising and falling hydrograph corresponding to storm periods. In WS 1 the average annual production of sediment after clear cutting was 230 t/km 2 /year roughly twelve times the pretreatment rate. Total sediment production rates increased initially, but leveled off after a few years at levels slightly higher than average. Total sediment yield for WS 3 was over four times that of WS 1 and 27 times that of WS 2. However, 88% of this delivery occurred in In December of 1964, a major storm triggered a series of debris slides probably accounting for most of the debris within a short time period (Grant and Wolff 1991). Figure 1. Map of lake sediment thickness (Ambers 2001). Conclusion In the Experimental Forest study each of the three watersheds behaved dramatically different in terms of magnitude and timing of sediment yield over the 30 year period. These differences can not simply be attributed to the specific harvesting conditions of the watersheds, but reflect the complexity between harvesting, major storms, and the inherent geological properties of the watersheds (Grant and Wolff 1991). The long term records of sediment production gathered from these three mountain watersheds reveals that sediment yields are highly contingent upon the interplay of numerous factors. Differing factors in proportion of unstable watershed area, drainage network morphology, and the volume of sediment stored in channels can be minute when examined individually, but together they are profound. The timing of harvesting in respect to large storms is another important factor influencing sediment yields in watersheds. Similarly, the results from Dorena Lake varied greatly depending upon hydrologic conditions. Large floods seemed to have a much more intense impact than the minor changes following episodes of logging. Provided that the fluctuations in precipitation and stream flow in this study encompassed several orders of magnitude, the minor changes in logging rates and practices over the 50 year period were virtually undetectable (Ambers 2001). Small watershed studies are perhaps to small-scale to get a true average reading and large watershed studies lack the resolution to differentiate between the multiple influences on sediment yield. The results of these studies revealed that logging does impact sediment transfer. However, the connection between the two is more complex than a simple linear relation. Many other factors must be taken into account to fully understand the dynamic relationship between logging and water quality. Thus, investigation of watersheds at multiple levels is needed to fully understand all of the natural and man induced changes in sediment yield.