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"ni~l!\ ')6 v:.... ~.... "- ... 2. A manometer similar to that described by Winter and others (1988) was used to measure the dif- ference in hydraulic head between ground water at a depth of I ft beneath the sediment! water internce in each piezometer and the river. The precision of the manometer readings was :to.075 in. This infonnation was used to calculate the hydraulic gradient between the ground water and surface water using equation I: i = dh/dl where i is the hydraulic gradient, unitless; dh is the difference in hydraulic head between ground water and surface water, in inches; and dl is the distance between the ground-water and surface-water measurement points, in inches. 3. A 4-in-diameter PYC tube was installed immedi- ately adjacent to each piezometer so that the bottom opening of the tube was at the same depth as the midpoint of the screen on the piewmeter. Riverbed sediments probably were disturbed as the PYC tube was installed. The top opening of the tube extended above the river surface, This PV C tube was used in a constant-head penneameter test to measure the hydraulic conductivity of the I-ft cylinder of sediment across which hydraulic gradient also was measured (Bowles, 1978). The tests assumed that all of the head loss occurred in the column of sediment tested and that head losses in sediment below the penneameter were neg- ligible. This assumption is realistic because vertical hydraulic-conductivity values in sediments usually are much lower than hori- wntal hydraulic-conductivity values. The penneameter test was run for I minute or until I L of water had passed through the scdiment column, whichever came first. The test was done in triplicate and the average value of hydraulic conductivity was used in further calculations. Hydraulic conductivity was measured each time the cross sections were sampled. 4. The hydraulic-conductivity and hydraulic- gradient measurements were used to cal- culate the specific discharge of ground water at each piezometer using equation 2: v = -Ki (2) where v is the specific discharge of ground water, in feet per second; K is hydraulic conductivity, in feet per second; and i is hydraulic gradient, unitless. 5. Instantaneous ground-water discharge at each cross section was calculated using equation 3: (1) q, = [L(v. L.)](5,280), n = piezometers I through 5 (3) where q, is instantaneous ground-water discharge for the cross section, in cubic feet per second per mile; v. is the specific discharge of ground water at the nth piezometer, in feet per second; L. is one-half the distance from the nth piezom- eter to each of the adjacent piezometers, in feet. In the case ofthe piezometers near the bank, L. is the distance between the bank and the nth piezometer plus one-half the distance from the nth piewmeter to the next adjacent piezometer, and 5,280 is the number of feet per mile. Note that the specific discharge calculated using equation 2 expresses an average rate offlow rather than the velocity of individual particles of water; therefore, the use of specific discharge in equation 3 allows for the calculation of an average instantaneous ground-water discharge rate. This rate will be referred to as the instantaneous ground-water discharge rate in the discussion to follow. Temporal variations in instantaneous ground- water discharge across the sediment!water interface were measured at the McKay Road, Henderson, and Road 8 cross-section sites using this technique by taking measurements hourly for 24 hours. Continuous River.Stage and Ground-Water- Level Measurements Continuous measurements of river stage and ground-water levels were made at the Henderson and Road 8 sites. River-stage measurements at the Henderson site were obtained from the long- tenn streamflow-gaging station at that site (station number 06720500) (pI. I). River-stage measurements at the Road 8 site were obtained from a stilling well using an automatic data recorder connected to a paper- METHODS OF DATA COLLECTION 7