SEDIMENTS IN THE DELTA OF ERASER RIVER 119 



The delta is building out into fairly deep water in the Strait of 

 Georgia in spite of the facts that the river is tidal for a considerable 

 distance above its mouth, with a mean tidal range of 6.4 feet and a 

 maximum range of 15 feet at its mouth, and that the seaward front 

 of the delta is swept by fairly strong tidal currents. The out- 

 building occurs because of the dominance of the river currents 

 over the tidal currents. The river currents are strong enough to 

 transport large quantities of coarse and fine material which the 

 tidal currents cannot transport into deep water. The tides and 

 tidal currents have the effect, however, of giving an unusual form 

 to the seaward portion of the delta. In its seaward portion the 

 river flows for several miles through sand banks exposed in large 

 part at low tide, but completely submerged at high tide. These 

 banks extend seaward on an average of 4 to 5 miles from the higher 

 delta land and are for the most part nearly level except where 

 channeled by outlet streams. The flood tides and tidal currents 

 tend to cause a diffusion and diversion of the river currents in this 

 part of the delta and prevent the river from building up its banks. 

 The coarse material in the sand banks is gradually moved seaward 

 by the dominance of the combined ebb-tidal and river currents, 

 which tend to carry it seaward, over the flood-tidal currents moving 

 inward. The fine material is kept in suspension until it is carried 

 seaward by the river currents and settles by gravity and through 

 the influence of sea water in causing it to flocculate, or is carried 

 inshore by the flood-tidal or wind-induced currents and lodges in 

 the grass-covered marshes and slack-water channels, tending to 

 build them up and extend the higher delta land seaward. The 

 flood-tidal currents also have the effect of giving to the greater 

 part of the subaqueous front of the delta a smooth, curved 

 outline lacking the finger-like projections characteristic of many 

 deltas. 



In structure, the delta presents the forms characteristic of a 

 high-grade delta built into fairly deep water. The fore-set beds 

 are well developed and extend from the 3 -fathom line to about the 

 30-fathom line and have an average dip of about lo*^'. The dip, 

 however, is irregular, and in places the subaqueous front of the 

 delta is nearly vertical for heights of i to 3 fathoms. Below the 



