I previously posted a link to the boundaries, but I didn't translate, the link has a map "cut out" that shows the Cuyuni Basin which clearly encompasses LC. Here are the boundaries:

Area Study
Geological aspects, climate and vegetation.
The River Basin Cuyuní
constitutes a part of the great
Essequibo River basin and covers the slopes
east of the Venezuelan Guayana,
approximately 4.2% of the country (Mago
- Leccia, 1970). Have wedge shape and
located primarily in the District
Roscio of Bolivar State, in the region
the "Land of Yuruari" (Vila, 1951),
between 6 º 00 'until 8 ° 00' north and
62 ° 33 'and 60 ° 20' west. The basin limits
to the north by the Sierra Imataca (mountains of
Nuria), south of the plateau
Gran Sabana (Sierra de Lema) and west
by mountains that separates it from the slope
the Caroní; it determines that all
the waters of the basin is Cuyuní
shed for the east and continue to
Guyana. The rivers in this basin have
a marked trend from north to south,
although rivers and Cuyuní chicane,
its highest portions, have direction
south-north.