528 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



This increased load has led the formerly almost limpid streams to 

 aggrade their lower reaches, to convert once fertile bottoms into 

 marshes or to cover them with sand and gravel. This aggrading, in 

 many instances, forced the streams to cut new channels or a network 

 of channels through the plain. But this lateral cutting is prevented now 

 by planting willows, aspens, balm of Gilead and other rapidly-growing 

 plants on the river banks down to the water's edge. The silt-laden 

 flood does little injury to these plants and the plain itself is injured 

 only by drowning of crops when floods come in the growing season. 

 Glenn contrasts conditions in the Coosa and Chattahoochee basins. 

 The former river rises in an area still forested and its waters in 

 flood carry little inorganic matter to cause destruction ; but forests 

 have been removed from the headwaters of the latter, floods are more 

 frequent and the accumulation of sand is very great — a condition 

 wholly unknown one hundred years ago. 



Rixon' gives similar testimony to the ability of humus to protect 

 itself as well as underlying material from erosion. " The litter and 

 underbrush among the alpine timber arc very heavy, having accu- 

 mulated for ages. One class of timber, having reached maturity, 

 decays, dies and falls, only to be supplanted by another growth, 

 which in time follows its predecessor." This is in a region where 

 rainfalls are infrequent but extremely violent. 



Tuomey^ studied the influence of forests on surface run-off. 

 His observations were made on four small catchment areas in south- 

 ern California, where wet and dry seasons are well-marked. In 

 December, iSqq, the rainfall was 18 inches. This was at the close 

 of the long dry season, when litter and soil alike were desiccated 

 and each absorbed a large part of the rainfall. The percentage of 

 run-ofl^ is given in the first column : 



35 

 33 

 43 

 95 



' T. F. Rixon, " Forest Conditions in the Gila River Forest Reserva- 

 tion," U. S. Geol. Survey, Prof. Paper, 39, 1905, p. 18. 



' J. W. Tuomey, " The Relation of Forests to Stream Flow," Year Book 

 of U. S. Dep't of Agriculture, 1903, pp. 279-288. 



12(> 



