NATURAL AREAS AND REGIONS 



613 



generally known as the Serrania del 

 Darien, reaches in many places an 

 altitude of 3000 to 5000 ft., but the crest 

 is interrupted at various points by 

 passes less than 1000 ft. high. Mount 

 Pirre is the name applied to a dominant 

 spur, slightly exceeding 5000 ft. in 

 altitude and projecting northward into 

 the Tuyra basin in a crescentic curve 

 with the axial trend of the continent 

 along the Panama-Colombian frontier. 



Aside from the higher mountain ranges 

 that form the Isthmian backbone, a 

 multitude of rather steep, extensively 

 eroded ridges separating narrow river 

 valleys ramify throughout the greater 

 part of the republic. Extensive and 

 fairly level plains occur at various 

 elevations, however, in the province of 

 Chiriqui, and along the Pacific coast 

 as far east as the Bayano River Valley. 



Owing to the narrowness of the Isth- 

 mus most of the rivers are short, and 

 from their sources, commonly interdigi- 

 tating along the opposite sides of the 

 deeply eroded continental divide, flow 

 directly to the sea, but there are several 

 notable exceptions. The greatest river 

 system of the republic is the Tuyra 

 Chucunaque. After draining a large 

 and very humid area, these two rivers 

 unite near the middle of the Isthmus 

 and in combination with several other 

 large streams pour an immense volume 

 of water into the Gulf of San Miguel. 

 The second river of the republic in point 

 of size is the Rio Bayano, which takes 

 a westerly course and joining the Rio 

 Mamoni, a much smaller stream, turns 

 southward and under the name of Rio 

 Chepo enters the Bay of Panama. The 

 most important river of the Atlantic 

 drainage is the Rio Chagres, whose 

 watershed is an interior basin. The 

 general course of the stream is westerly 

 to a point near where it enters the Canal 

 Zone and bends north to the Caribbean 

 Sea. The Chagres, whose waters are 

 now impounded in Gatun Lake, 164 

 sq. mi. in area, furnishes the water for 

 operating the locks of the Panama Canal, 

 and through the locks at the southern 



end of Gailliard Cut a part of its flow 

 is diverted into the Pacific Ocean. 



B. Climate 



While climatic conditions vary con- 

 siderably in different parts of Panama, 

 the region as a whole is subject to the 

 influence of two annual seasons, the 

 duration of which are correlated with 

 the direction of the prevailing winds. 

 During the so-called "dry" season the 

 northeast trade wind, blowing daily 

 from about the month of December to 

 the month of May, at times with con- 

 siderable violence, is accompanied by 

 light, but not infrequent precipitation 

 along most parts of the Atlantic slope. 

 At this season rather light cloud forma- 

 tions discharge their moisture along the 

 northern side of the Isthmus, the rain- 

 fall of the coast depending in a measure 

 on the height and proximity of the moun- 

 tains. At the higher elevations fogs 

 are very prevalent, condensed moisture 

 drips freely from the trees and tropical 

 rain forest conditions prevail. The 

 Pacific coast in marked contrast, at this 

 time has a true dry season, during which 

 little rain falls. During the wet season, 

 beginning usually about the latter part 

 of May and ending about the first of 

 December, southerly winds become 

 dominant and rains are general through- 

 out the Isthmus. 



At the Canal Zone, which is a cross- 

 section of the Isthmus about 50 mi. 

 in extent, the annual rainfall on the 

 Atlantic coast is nearly double that on 

 the Pacific coast. Official records for 

 1909 show a total rainfall of 93.06 in. at 

 Balboa, and 183.41 in. at Cristobal; 

 but the average for 13 years at the 

 former station is 71.67 in., and for 40 

 years at the latter station 130.03 in. 

 This relative difference in humidity of 

 the two sides probably obtains as far 

 west as the Costa Rican frontier, but 

 in eastern Panama the difference is less 

 marked. In much of the Darien region 

 the total rainfall is increased to an 

 annual precipitation of perhaps more 



