1 64 SEA AND LAND 



the nuul which they abundantly contain therefore settles 

 upon the bottom of the sea. Its precipitation is favored by 

 a mixture of the salt water with that which has come from 

 the land, for, as is well known, the infusion of a certain 

 amount of salt with water containing- suspended mineral par- 

 ticles hastens the |)rocess b)' which the sediments separate 

 and fall to the bottom. i\s long- as conimercial ships were 

 of small size, drawing- no more than twelve or fifteen feet of 

 water, delta ports of this class were generally suitable for 

 commerce ; but with the increase in draft of our modern 

 ships, these hax'cns require engineering v/ork to make them 

 fit for the needs of shipping-. 



Another disadvantageous feature exhibited by delta ports 

 is found in the frequency with which the principal path of 

 exit of the river waters changes its position. By reference to 

 a map of the Mississippi delta or those about the mouths of 

 other great streams, it will be seen that the river commonly 

 has a number of sei)arate arms by which its llow may escape 

 to the sea. In most cases detailed study shows that only one 

 of these exits is at any particular time the seat of the prin- 

 cipal stream. The others have been temporarily abandoned, 

 but may from time to time be reopened l)y the varying move- 

 ment of the waters. Hydrographers who have studied the 

 laws of river movement recognize the fact that these delta 

 channels are naturally, indeed we may say necessarily, subject 

 to changes in jjosition. When after a time an outlet has 

 extended in the manner of the present channel of the Missis- 

 sippi for a considerable- distance seaward, the energy of the 

 current is slackened b\- the lengthening of the slope over 

 which it moves, and so the flood waters begin to rise to a 

 greater height in the up-river portion of the delta than they 



