NATURAL AREAS AND REGIONS 



675 



with such regions. Between the annual 

 high and low water limits, lie immense 

 flood plain areas of perennially enriched 

 land locally known as "varzias." Nu- 

 merous lakes, natural canals both tem- 

 porary and permanent dot and intersect 

 this region. The whole basin has been 

 graphically likened to that of a region 

 recently the bottom of a great sea, 

 which, gradually subsiding, leaves ridges 

 of alluvium, cut off lakes, networks of 

 intersecting waterways and small and 

 large areas that have been covered with 

 water only shallowly. The coastal 

 plain of this basin is low, swampy and 

 mangrove embowered. 



2. Climate 



From the standpoint of climate, the 

 area can be divided into at least four 

 regions: the region of perpetual snow, 

 the temperate, the subtropical and the 

 tropical: and two seasons, wet (winter) 

 and dry (summer). The region of 

 perpetual snow (above 17,000 ft., aver- 

 age temperature about 1°C.) occurs only 

 in connection with the eastern flanks 

 of the high Andes. The temperate 

 lands lie in this same region roughly 

 above 9000 ft. with an average annual 

 temperature of about 12°C. The sub- 

 tropical comprises such regions as the 

 Yungas valleys of Bolivia, lying at an 

 elevation of between 3000 and 9000 ft. 

 (average annual temperature 21 °C.), 

 while the remainder of the area, making 

 up most of the basin, is tropical (average 

 annual temperature about 25°C.) . Each 

 of these regions is characterized by its 

 own faunal and floral types. The 

 seasonal rhythm differs over this area. 

 North of the Amazon the seasons are 

 retarded, while south of it they are 

 advanced. On the north, on the Rio 

 Negro, the rainy season begins in Feb- 

 ruary, reaches its maximum in April, 

 May and June. On the Rio Branco, a 

 tributary of the Rio Negro, the rains 

 begin in April and reach their maximum 

 in June and July and end in August. 

 In the Acre district on the south side, 

 the wet season, or tiempo de agua lasts 

 from September to April, the driest 



months being June and July. On the 

 Beni and Mamore rivers, the rains are 

 expected toward the end of November, 

 February and March being the wettest 

 months, with June marking the begin- 

 ning of the dry season {tiempo de secho). 

 Portions of the main river never have 

 very marked seasonal distinctions. At 

 Pebas, below Iquitos, Peru, the driest 

 month is August, while at Para (Belem) 

 the rains continue throughout the year, 

 September to November being the 

 months of least precipitation. The 

 average annual rainfall at Para is 248.6 

 cm.; the total average number of rainy 

 days based on 18 years of records, 252, 

 the months of January to June ranging 

 from 22 to 28 days per month. At 

 Manaos (based on 5 years of records) 

 the average annual rainfall was 165.7 

 cm., rainy days per year, 154.3, the 

 rainiest months by days being December 

 to May ranging from 19.3 to 22.7 days 

 per month. At Porto Velho on the Rio 

 Madeira the average annual precipita- 

 tion (years 1907 to 1912) was 264.0 cm., 

 ranging from 14.22 cm. in September 

 to 38.68 cm. in December with 1.78 cm. 

 in July, the driest month. At Para, 

 most of the rain falls in the afternoon, 

 evening or during the night. In the 

 Beni river country, in Bolivia, this also 

 seemed to be the case. The relative 

 annual humidity at Para is 88.3, maxi- 

 mum 89, minimum 43, the highest month 

 being February (93.8), the lowest Oc- 

 tober with 84.2 At Manaos, the figures 

 are: average 77.6, maximum 99, mini- 

 mum 54. The highest humidity at this 

 station occured at 7 a.m., the lowest at 

 4 p.m. The average annual temperature 

 at Para (Belem), based on 9 years records, 

 was 25.8°C., with but slightly over a 

 degree difference between the monthly 

 averages. Another set of observations 

 indicates only 3° or 4° difference between 

 the maximum and minimum over a 24 

 hour day, the highest temperature being 

 recorded about 1 p.m. At Obydos sev- 

 eral hundred mi. from Para, essentially 

 the same temperatures are recorded. 

 At Porto Velho, the average recorded 

 temperature was 27.78°C., maximum 



