304 The National Geographic Magazine 



it follows that many interfering condi- 

 tions may arise, tending to retard or 

 accelerate the passage of the crest of 

 the flood wave. No absolute rule is, 

 therefore, possible ; but the forecasting 

 of the exact flood stage many days, or 

 even weeks, in advance at important 

 river stations is of such frequent occur- 

 rence as to indicate that, although the 

 forecasts are empirically made, they 

 have a substantial commercial value. 



Each forecaster in charge of a river 

 center has a definite section of the river 

 system to watch and for which he must 

 forecast. He receives the necessary 

 telegraphic reports of the daily rainfall 

 that has occurred over the tributaries 

 to his river district, reports of the gauge 

 readings nearer the source of the main 

 river than his own station, and gauge 

 readings from many of the tributary 

 streams. He is familiar with the area 

 of the catchment basin from which his 

 rainfall reports are received, the con- 

 tour and configuration of the surface, 

 and the permeability of the soil. A 

 slowly falling rain of considerable 

 amount on a nearly level and permea- 

 ble soil may cause little rise, while a 

 rapidly falling rain of the same amount 

 on an impermeable and greatly inclined 

 surface will gather quickly in the chan- 

 nels of the tributaries and soon become 

 a rushing torrent in the main stream. 

 It is thus seen that many modifying 

 conditions must be taken into consider- 

 ation. The forecaster studies the his- 

 tory of previous floods under various 

 temperatures and absorptive conditions 

 of soil. He knows that the rainfall 

 may be augmented by the melting of 

 snow, if any there be on the ground, 

 and that the temperature is an impor- 

 tant factor in the flood ; that on a 

 frozen soil, under moderate heat, the 

 entire precipitation, plus meltage, may 

 flow away without appreciable absorp- 

 tion or evaporation and create higher 

 water in the rivers than would be the 

 case if the soil were open, and that an 



unfrozen but saturated soil presents to 

 the flowing water practically the same 

 surface, so far as the latter affects the 

 flood, as a frozen soil. Of the precipi- 

 tation that is absorbed a part is evap- 

 orated, a part taken up by vegetation in 

 making its growth, and the remainder 

 sinks to the impervious rock, which 

 lies at no great depth below the surface. 

 It slowly follows the slope of the rock, 

 and gives rise to the springs that sup- 

 ply the steady flow of the streams and 

 rivers. This portion of downpour, 

 while unimportant in the causing of 

 floods, needs to be considered by the 

 river forecaster, for an abundance of 

 well-absorbed rains during the spring 

 and early summer means the mainte- 

 nance of fair stages in navigable rivers 

 during the usual low- water season, and 

 forecasts of low-water stages are nearly 

 as important to commerce as the pre- 

 diction of flood heights. In brief , floods 

 have their origin in the surface dis- 

 charge, while the low-water flow of 

 streams is mainly due to the under- 

 ground waters. 



The zero of a river gauge is placed at 

 the level of the lowest water known, 

 and if at any subsequent time a stage 

 still lower is recorded it is read as a 

 minus quantity. The danger line varies 

 with the locality. On the Ohio river, 

 on account of its narrow channel and 

 its precipitous banks, the water must 

 show vertical rises varying from 30 to 

 50 feet before the danger line is reached. 

 At Cincinnati the danger line is 45 feet 

 above the zero of the scale, and a height 

 of 71 feet has been recorded. On the 

 upper Mississippi the danger lines aver- 

 age about 15 feet above zero, but from 

 St Louis to Vicksburg they average 

 about 35 feet, while at New Orleans the 

 danger limit is but 13 feet above zero. 

 An impermeable ground, such as that 

 over granite bed rock, is marked by 

 many rivulets and streams in compari- 

 son with the number that are found in 

 a permeable soil of equal rainfall . When 



