More than 50 years of daily stream-discharge records are available for the Cowlitz River at Castle Rock. This station is 2.7 mi downstream from the mouth of the Toutle River. Depositional effects of the May 18, 1980, Toutle River lahars on flood elevations were evaluated from existing cross-section data. When sediment sampling began on May 19, 1980, bed elevation at the station had increased by more than 10 ft with sediment deposition (Lombard and others, 1981). The Cowlitz River channel was dredged day and night for several months to restore flow capacity in the inundated reach. River depth, sediment discharge, and bed and flood elevations were regularly monitored at Castle Rock to maintain flood preparedness. Discharge measurements were made weekly or more often when needed. Observer samples were collected daily at a single sampling vertical on the bridge. Records of daily values at the Cowlitz River at Castle Rock end with water year 1984 ( fig. 41 ).
Annual sediment discharges at the Cowlitz River at Castle Rock were lower than at the Toutle River at Highway 99 during water years 1980-82. Differences in daily sediment discharge between the two stations are plotted in figure 42 . Comparison of sediment-discharge records shows that the losses in sediment discharge occurred on days of storm flow and were not matched by gains in sediment discharge on days following the storm flow . Deposition of sediment at the mouth of the Toutle River, and the transport of coarse sand in the Cowlitz River as bedload rather than suspended load, are possible reasons for the differences.
Sediment data also were collected at sites in the Cowlitz River basin upstream from the Toutle River (Dinehart and others, 1981). The Cowlitz River at Packwood, the Cispus River near Randle, and the Cowlitz River at Toledo are long-term gaging stations at which sediment was sampled periodically in 1980 and 1981. Daily sediment discharges, based on daily observer samples, were computed for June through September 1980 for the Cowlitz River at Packwood and Cispus River near Randle. Both sites were affected temporarily by ashfall. Periodic sediment samples (pre- and post-eruption) were collected at the Cowlitz River at Toledo (16.3 mi upstream from Castle Rock) and the Cowlitz River at Kelso (12.4 mi downstream from Castle Rock).
