2. Eagle River and Ship Creek. 



These rivers are tidal in their lower reaches. Eagle River (Fig. 5), which 

 originates at Eagle Glacier, carries a large suspended load at a mean summer 

 discharge of 30 cubic meters per second. Ship Creek carries much less sediment 

 at an average summer discharge of about 3.2 cubic meters per second. In calcu- 

 lating the water discharge from these rivers during an ebb cycle, the average 

 river discharge was added to the discharge resulting from the water volume of 

 the cross section created as a result of the previous floodtide. This volume, 

 calculated using survey data, hydrographic charts, and 1:80,000 topographic maps, 

 is equal to the difference in river channel volume at high tide and at low tide. 

 The total ebbtide discharge results are in Figure 4. In Eagle River, the total 

 ebbtide discharge includes 70 percent from tidal flow and 30 percent from river 

 discharge. The ratio for Ship Creek is 64 and 36 percent, respectively. Eagle 

 River has a particularly extensive tidal reach near its mouth (Fig. 5). 



Figure 5. An aerial view, east from Knik Arm, of mouth of Eagle River. 

 3. Dillingham Harbor . 



Dillingham Harbor is a "half-tide" type harbor (Everts, 1976). Because of 

 currents resulting from the high tides, as well as severe winter ice conditions, 

 half -tide harbors are constructed as enclosed basins sited adjacent to, rather 

 than within, navigable estuaries. Harbor depths are generally specified near 

 mean lower low water (tILLW) to reduce initial excavation costs. The unique fea- 

 ture of these harbors is a sill at the harbor mouth designed to maintain ade- 

 quate water levels during low tides. The sill provides flotation for vessels 

 during low tidal stages, and it restricts navigation into or out of the harbor 

 to times of higher tidal elevations. 



13 



