24 
KIRTLEY F. MATHER 
ping were perforce such as would not delay progress. Trans- 
portation was by means of dugout canoes, propelled and piloted 
by Yuracares Indians. Directions were determined by frequent 
readings of a Brunton compass; distances were approximated by 
noting the interval of time which elapsed while the canoe was 
on the determined bearing. The speed of the canoe had been 
previously ascertained by timing it over a marked course, and, 
although the methods used in making the map were obviously 
crude and subject to considerable error, it is believed that the 
results obtained in the main are fairly accurate. 
Rio Secure, above the junction with Rio Isiboro at Puerto 
Calvimonte, averages a hundred yards in width. It is a brown, 
silt laden stream, sliding swiftly between banks of clay or sand, 
capped with six or eight feet of rich black soil, which rise abruptly 
from the water’s edge to the level of the jungle covered plain. 
At low water these banks are 15 to 25 feet, high, but in the rainy 
season the river brims its banks and floods the ground between 
the trees of the tropical jungle. The Isiboro is nearly as large 
as the Secure above the junction of the two. The Secure’s vol- 
ume is therefore nearly doubled at Puerto Calvimonte. From 
that point to its mouth the river averages a little over 150 yards 
in width. Rio Mamore is a much larger stream, with a width of 
at least a quarter of a mile at low water, between the mouth of 
the Secure and the vicinity of Trinidad. A detailed map of the 
lower part of the Secure and a portion of the Mamore forms 
figure 2. Throughout the area of this map and for many miles 
upstream and downstream beyond it, the gradient of the Secure 
and Mamore is practically constant, less than half a foot per mile 
of river course. 
The meander patterns of the rivers, shown in figure 2, are 
characteristic; curve follows curve in dizzy succession along the 
tortuous stream course. The typical meander curve is not an 
arc of a circle, but is formed of short sharp bends alternating with 
long, comparatively straight stretches. Many times while 
paddling steadily downstream we found ourselves within a hun- 
dred yards or so of the place where we had been an hour before. 
Occasionally we noted cut-offs where during the previous rainy 
