In September and October 1972, large depth changes in the main axis 

 o£ the southern branch (Fig. 3) coincided with the slumping of the 

 shallow-water terraces above the north wall and at the end of Moss Land- 

 ing Pier, and activity on the southern wall of the main axis shoreward of 

 the submarine ridge (Fig. 4). The probable sequence was: (a) Shallow 

 areas shoaled in calm summer months and sediment accumulated on the walls 

 and especially in the channels, and (b) rough seas caused sediments to 

 move down channel, stimulating slumps in deeper channels. The main axis 

 accumulated more sediment, received more input from shallow water, and 

 changed in depth more than the other channels. 



Sediment input at the canyon head was primarily from longshore drift 

 and tidal scour of the Elkhorn Slough and Moss Landing Harbor. Fragments 

 of thallophytic algae accumulated all year but input was highest in the 

 summer. In June and July 1971, the main axis floor contained a 0.3-meter- 

 deep mud layer; algal fronds were half-buried in the sediment. Oxygen 

 concentration was low and the hydrogen sulphide odor was strong in water 

 samples taken 1.5 meters above the substrate. By September the fronds 

 had broken into many smaller pieces and were mixed into the sediment. 

 In December 1971 the floor was covered with coarse sand. 



Further evidence of activity during most of the year came from obser- 

 vations of dredged material deposition. Material was piped to the head 

 of Moss Landing Pier at three different times during the year. Coarse 

 sediments accumulated in a pile directly below the end of the pipe, then 

 slumped into the canyon. During a previous monitoring (Harville, et al . , 

 1968) , the dredged material had been dumped in August 1967 and the piles 

 had moved completely into the axis by December 1967. Diver observations 

 revealed a mound of sediment in the axis at a depth of about 30.5 meters. 

 Material deposited in November 1969 and April 1972 appeared to move with- 

 in 1 to 2 months. 



Shepard (1948) concluded that there was little change in the major 

 topography of the canyon head since earlier soundings were made by the 

 U.S. Coast and Geodetic Survey in 1933. Observations in the southern 

 branch indicated no major changes in the topography described by Shepard. 

 Thus, main topographic features have appeared stable for the last 40 

 years. 



Summary 



a. Sediment and algae accumulation was highest during the calm 

 summer months . 



b. Sediment movement increased in the fall and continued through 

 the winter into early spring. 



c. Accumulation was low in the winter; activity was high. 



d. The largest topographic changes occurred during the fall with 

 the first rough seas. 



14 



