INTRODUCTION. 



GENERAL TOPOGRAPHY AND HYDROGRAPHY OF PANAMA. 



In the region of the Canal Zone the Atlantic slope is very much 

 broader than the Pacific slope. Following the line of the Panama 

 Canal, it is about 40 miles from the Atlantic coast to the crest of the 

 divide and only 10 miles from thence to the Pacific coast. This, in 

 the Canal Zone region, where the rainfall is heavy,* gives rise to the 

 comparatively large Rio Chagres on the Atlantic slope opposed by 

 rather small coastal streams on the Pacific slope, namely, the Rio Chor- 

 rera, Rio Grande, Rio Marte Arnade, Rio Abaco, Rio Juan Diaz and 

 other small streams. 



Going eastward or toward Colombia the Pacific slope becomes 

 broader and the Atlantic slope narrower. The increased width of the 

 Pacific slope then gives rise to the rather large Rio Bayano, the mouth 

 of which is about 30 miles east of Panama City, and to the still larger 

 Rio Tuyra which empties into San Miguel Bay, about 100 miles south- 

 east of Panama City. Opposite these streams, on the narrow Atlantic 

 slope, there are only very small coastal streams. 



In western Panama the watershed more nearly follows the median 

 line of the Isthmus and no large streams occur on either slope. 



There are then three river basins of considerable size in Panama, 

 viz.; the Rio Chagres on the Atlantic slope and the Rio Bayano and the 

 Rio Tuyra on the Pacific slope. Collections were made in all of these 

 streams. The Rio Chagres with its numerous tributaries was quite 

 thoroughly explored, and it is believed that our collections contain, 

 with perhaps very few exceptions, all the species occurring there. Our 

 explorations in the Rio Bayano and the Rio Tuyra basins were not so 

 thorough, and our collections from these rivers are certainly not ex- 

 haustive, but sufficient to indicate the nature of their faunas. Besides 

 these large streams, the small streams opposite the Rio Chagres and 

 as far westward as the Rio Chame on the Pacific slope were visited and 

 collections made in each. On the Atlantic slope only a few of the small 

 coastal streams, two at Porto Bello and two at Toro Point, were visited. 

 This then leaves many small streams of western Panama unexplored. 



"The rainfall in the Canal Zone region is heaviest on the Atlantic slope and 

 particularly at Porto Bello where during a single day of 24 hours as much as n 

 inches of water are known to have fallen. In 1909, 58.07 inches of rain fell during the 

 month of December and the total rainfall for the year was 237.28 inches. 



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