10 to 50 percent and varies seasonally. Shepard (1948) observed a 50- 

 percent slope from a similar shallow sandflat on the north wall of the 

 main branch (Fig. 1) . 



A Kaiser Company submarine outfall was placed parallel to the terrace 

 along the edge of the north submarine ridge (Fig. 2) . Trenching opera- 

 tions exposed an underlying, consolidated silt-clay material to depths of 

 6.1 meters below the existing bottom in some areas. In the main axis, 

 several silt-clay outcrops were present on the steep part of the north 

 wall. These outcrops contained a number of living burrowing bivalves, in- 

 dicating that the outcrops were usually free of sand cover. Vertical 

 walls of consolidated silt-clay material were also exposed in the main 

 axis after a sediment slump in October 1972 (Fig. 3). Thus, the head of 

 the southern branch appeared to be formed of a consolidated silt-clay 

 material covered to varying depths by fine sand. 



Observation dives and sample collections were made between June 1971 

 and November 1972, using scuba and underwater diver vehicles (Farallons). 

 Underwater visibility (0 to 1.2 meters) was usually poor. Maximum infor- 

 mation was obtained on the few days that visibility increased 3 to 7.6 

 meters. 



In late spring of 1972, permanent transect lines were established to 

 detect changes in gross topographic features and sediment movement. 

 Transects were marked at 9.1-meter intervals between permanent stations 

 by 2.4-meter fence anchors (Slattery and Oliver, 1972), and the distance 

 from the top of the anchor to the bottom was periodically measured. 

 Figure 2 was compiled from the diver observations and measurements made 

 along these transects from April to October 1972. 



Seasonal physical changes of the canyon axis and the south canyon wall 

 were of special interest. To test the dynamic conditions present in the 

 axis at a 30.5-meter depth, five 0.6-meter fence anchors were arranged in 

 a straight line across the 10.7-meter width (Fig. 2, line C-3 to P-3), and 

 oriented normal to the canyon walls. To test the southern slope condi- 

 tions, a similar transect was placed at right angles to the contours of 

 the steep slope south of the P-3 transect line (Fig. I) . 



The greatest changes in topography occurred along the shallow-water 

 terraces (6.1 to 12.2 meters) and in the channeled areas. Submarine 

 ridges and deeper canyon walls were more stable. 



Water depths at the head of the Moss Landing Pier changed from 6.1 

 to 7.6 meters in the summer of 1972, and 10.7 to 12.2 meters in the fall 

 and early winter. In the summer the bottom under the pier consisted of 

 coarse sand; in the fall the surface sand moved away from the pier. The 

 slope from the end of the pier to the flat part of the canyon floor was 

 considerably steeper during the summer. In early fall of 1972, three 

 sand terraces, 0.6 to 0.9 meter high, about 15.2 to 30.5 meters apart, 

 and arranged perpendicular to the main axis, occurred along this slope. 

 The activity around the Moss Landing Pier indicated periodic migration 

 of large amounts of shallow-water sediment into the main axis of the 

 canyon head. 



