124 ECONOMIC BOTANY OF ALABAMA. 



Topography and hydrography. — The whole area of 

 course lies close to sea-level, with a gradual ascent to the 

 northward. At any particular latitude the banks of the 

 channels are the highest points, and at the upper end of 

 the delta these may be 15 or 20 feet above sea-level. 

 The channels form an intricate network, and the largest 

 ones are the Mobile River on the west side and the 

 Tensas (or Tensaw) on the east. Both are navigable, 

 but the Mobile River is the principal one used by steam- 

 boats going up into the interior. The water is always 

 more or. less muddy, but it cannot fluctuate much with 

 the seasons, on account of its nearness to the Gulf, At 

 the head of the bay or mouth of the rivers the seasonal 

 fluctuation is of course almost nothing, or less than the 

 semi-diurnal tidal fluctuation, while at the head of the 

 delta the river may rise as much as 10 or 15 feet, and 

 the tide is perceptible only when the water is pretty low. 



Climate. — The climatic data for Mobile, at the south- 

 ern end of the delta, are probably sufficiently representa- 

 tive for the whole area, and these can be found in the 

 appendix. As far as the water-level in the swamps is 

 concerned, that of course depends less on local climatic 

 conditions than on the rainfall in the interior of the 

 state. 



Forest types. — It has been found that the amount of 

 seasonal fluctuation of water is a factor of fundamental 

 importance to swamp vegetation; and the local distribu- 

 tion of trees in the delta can be correlated best with this 

 factor. Near the mouths of the rivers several trees 

 abound which are totally absent from the alluvial 

 swamps farther inland, but are common enough in the 

 non-alluvial swamps of smaller streams in the neighbor- 

 ing pine hills. At any given latitude in the delta the 

 seasonal fluctuation is greatest along the channels, and 

 the trees more characteristic of alluvial swamps reach 

 their coastward limits in such situations. At the very 

 extremity of the delta, where the seasonal fluctuation is 

 negligible, conditions are unsuitable for trees, and the 

 vegetation is mostly marsh instead of swamp; perhaps 

 because trees cannot germinate in soil perpetually sub- 

 merged. 



