THE MEANDER PATTERNS OF RIOS SECURfi 
AND MAAIORfi, EASTERN BOLIVIA' 
KIRTLEY F. MATHER 
The greater part of northeastern Bolivia is a lowland plain 
drained by a network of streams which unite to form Rio Madeira, 
one of the larger tributaries to the iVmazon. Between Guajara 
Mirim and Porto Velho (see fig. 1), the Madeira crosses the 
northwestern extremity of the Pre-Cambrian shield of BraziP 
in a series of cataracts and rapids. The altitude of the river bed 
at the first of the cascades, close to Guajara Mirim, determines 
the level of the floor of the entire basin stretching from that 
point to the eastern foot of the Andes of Bolivia and Peru. The 
streams from the mountains debouch upon this lowland, heavily 
laden with silt, and the plain is in consequence a plain of aggra- 
dation. Its surface is between 600 and 1000 feet above sea level, 
and slopes with remarkable uniformity and exceedingly low 
gradient from the foot hills of the Andes to the head of the cata- 
racts of the Madeira. Because of the uniformly low gradient 
the streams which traverse this featureless plain for hundreds of 
miles have developed meanders of unusual complexity and per- 
fection. The opportunity which they afford for the study of old 
age stream patterns is excellent. 
While engaged in geological explorations for Richmond 
Levering and Company in August and September, 1920, I crossed 
the eastern Andes from Cochabamba to the headwaters of Rio 
Chapare, and travelled overland in the foothill region to Rio 
Secure.^ After a brief examination of the upper reaches of this 
1 A paper presented before the Section of Geology of the Ohio Academy of 
Science, April, 1922. 
2 Branner, J. C., Geol. Map of Brazil, Bull. Geol. Soc. of Amer., vol. 30, 
plate 7, 1919. 
3 Mather, K. F., Explorations in the Land of the Yuracares, Eastern Bolivia; 
Geographical Review, vol. 12, pp. 42-56, 1922. 
22 
