Three figures are included to illustrate variability and operational consequences. Figure 14.1 shows a portion of a tree ring growth index time series from Dell, Montana (Lettenmaier and Burges, 1978). The time between A and B is 50 years, a typically data rich record length when determining system reliability. One form of variability is an apparent increasing trend within this 50-year period. When viewed in a larger time frame, it is clear that there is no trend but the series indicates different types of variability—extreme swings from low to high as well as an apparent increasing trend superposed on extreme swings. Tree-ring series are being used increasingly in regression equations as surrogates of streamflow that occurred before stream gauges were installed with the objective of providing an equivalent long historical record of "streamflow that might have occurred."