233 



high water and the turn of the current is then </>/». If then the Dela- 

 ware were an ideal estuary, the primary current at any station would 

 turn from flood to ebb 37°.35/28.98=1.29 hours after high water. 

 The actual currents turn in this direction from an hour to an hour and 

 a half after high water at the station. 



The general characteristics of the tides and currents in the tidal 

 portion of the Delaware conform therefore to those deduced for an 

 ideal estuary. 



446. Prevalence of estuaries of typical for m. — The depth of a natural 

 tidal channel is far from constant, and the variation in its width which 

 would be required to produce currents of constant strength departs 

 somewhat from that of the ideal estuary deduced in the preceding 

 paragraphs. However, as a natural channel carrying a constant steady 

 flow tends toward a general uniformity of width, a tidal estuary tends 

 toward the funnel-shaped form of an ideal estuary. In a tidal channel 

 which has not such a form, the currents have different strengths from 

 section to section and the bed tends to scour where the currents are 

 the stronger, and to fill where they are the weaker. Tidal channels in 

 alluvial material therefore mold themselves into the typical estuary 

 shape. The result of this process is strikingly shown in the natural 

 channels through the tidal mud flats bordering a sheltered bay or 

 coastal sound, in which the wave action does not cause enough littoral 

 drift to contract the outlets. A glance at a chart of such a region, or a 

 view from the au', shows that the many channels cut through these 

 flats by drainage from the uplands have molded themselves into the 

 typical estuary form, generally with a sinuous alignment. 



447. Large rivers which enter the sea through an alluvial coastal 

 plain also usually cut for themselves a typical estuary channel; unless 

 they carry down silt and sand at a faster rate than can be molded by 

 the tidal currents, when they maintain a generally uniform cross 

 section to an ever-growing delta at their junction with the sea. A 

 delta generally is found at the mouth of a silt-bearing river which, 

 like the Mississippi, enters a sea having a small tidal range; but the 

 burden of detritus may be sufficient to form a delta at the mouth of a 

 river where the tidal range is large. Thus deltas are found at the 

 mouths of the heavy silt-bearing rivers which enter Puget Sound, 

 although the diurnal tidal range in the sound is generally 10 feet or 

 more. 



448. Many rivers enter the sea through submerged valleys, which 

 ordinarily widen toward the sea and have the general shape of a self- 

 made estuary. If the valley has become filled with alluvial deposits of 

 fairly uniform consistency, the tidal flow generally ha,s molded an 

 estuary of typical form, often subdivided by islands and shoals. 

 The entrance to an estuary from the open sea usually is contracted 

 by deposits from the littoral drift along the coast line; but this contrac- 



