AQUEOUS AGENCIES. 37 



sissippi delta is pushing seaward more rapidly than any 

 other,, evidently because it pushes along narrow lines. It 

 is advancing now at the rate of three hundred and thirty 

 feet per annum, or a mile in sixteen years, or six miles 

 per century. The age of deltas can not, however, be got 

 in this way, because the river often changes its mouth, 

 and dumps its freight now here, now there, along the 

 whole water-front of its delta. The age or time is usually 

 estimated by getting the cubic volume of the delta, and 



dividing this by the annual mud-discharge (T = - -T J. 



But without more accurate observations than have yet 

 been undertaken, these estimates are not entitled to much 

 confidence. 



5. Estuaries. 



The wide mouths of certain rivers are called estuaries. 

 We have already explained why some rivers have estuaries 

 and some make deltas. All the rivers running into the 

 Atlantic and Pacific Oceans have estuaries, because the 

 tidal currents are stronger in carrying away than the river 

 in bringing down and depositing sediments. The Bay of 

 Fundy, the Hudson Eiver to near Albany, the Delaware 

 and Chesapeake Bays, Albemarle and Pamlico Sounds, 

 the Bay of San Francisco, and the Lower Columbia River 

 are estuaries. So also are the wide mouths of the Amazon 

 and La Plata Rivers. So also the firths of Scotland and 

 the fiords of Norway. The velocity and therefore erosive 

 power of tides in estuaries is sometimes enormous. The 

 trumpet-shaped mouth of the river takes in a large mass 

 of the tide-wave. As this passes up it is compressed into 

 a narrower channel, and therefore rises higher and rushes 

 with increasing velocity. In the upper part of Bristol 

 Channel the tide rises forty feet ; in the Bay of Fundy, 

 sixty feet ; in Puget Sound, twenty feet. If the water be 

 shallow and the resistance to advance great, the tide rises 



