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Fool Creek, the small (714 acre) experimental watershed on the Fraser <br />Experimental Forest, described earlier, represents one of the most definitive <br />data sets world -wide; documenting both the initial effect of timber harvest <br />on increasing water yield, and the more subtle reduction in the initial <br />increase in water yield that occurs as the vegetation recovers or re -grows <br />over time. Figure 9 represents a double -mass plot (Anderson 1955) of <br />cumulative flow from Fool Creek (the treated watershed) plotted over the <br />cumulative flow from East St. Louis Creek (the control watershed). The <br />data for the period 1943 -1955 (Point A to B on figure 9) represents the pre - <br />harvest, or calibration, period and as can be noted, the relationship is linear <br />(follows a very straight line), and a regression line has been fitted to the <br />calibration data and extended to point C (equality, or a 1:1 relationship, is <br />not required). In 1954 -1956, 40 percent of the watershed area was harvested <br />on Fool Creek and resulted in an average increase in flow of 40 percent. <br />The increase in flow from Fool Creek began immediately. This abrupt <br />change in the relationship between Fool Creek and East St. Louis Creek, as a <br />result of the increase in flow from Fool Creek, can be evidenced by the <br />change in the trajectory of the double -mass plot at point B. Flow increased <br />on Fool Creek (Y -axis) relative to that for East St. Louis Creek (X -axis) <br />causing the relationship to deflect upward. Since 1956, the clear cuts on the <br />Fool Creek watershed have gradually recovered naturally. By 1984, <br />approximately one -third of the original vegetation biomass had returned. By <br />1995, approximately one -half or more of the original increase in flow had <br />been lost as a result of regrowth. Regrowth can be documented by the <br />gradual, almost imperceptible, arc present in the double -mass plot between <br />points B and C (figure 9) as the flow returns to the relationship that existed <br />prior to harvest. Eventually, we would expect the double -mass plot to <br />parallel the regression (solid) line fitted to the pre - harvest data. If the <br />recovery- line "over compensates ", it would imply the younger, more <br />vigorous stand is using more water than the original stand. <br />The Fool Creek watershed experiment sets the standard in the snow zone for <br />hydrologic comparisons that document the effect of forest disturbance on <br />stream flow response both in terms of initial response and the recovery. <br />Data from highly controlled experimental watersheds, such as Fool Creek, <br />are usually more definitive in demonstrating response than are landscape <br />scale watersheds, normally monitored with less rigor and for largely <br />forecasting purposes. <br />1 <br />1 17 <br />