The substrates of the Llanos region of the Orinoco basin include various combinations of peat, mud, fine sand, and clay. Colors include black, brown, red, or white. Fallen branches, roots, leaf litter, and palm fronds are common.
Here is a list of commonly found aquatic plants that may be found in the region. I assembled this list using a highly trustworthy source. Yes, that's right, the Internet.
Common Amazon Sword (Echinodorus amazonicus)
Pygmy Chain Sword (Echinodorus tenellus)
Anacharis (Egeria densa)
Pennywort (Hydrocotyle leucocephala)
Pipewort (Eriocaulon sp.)
Variable Leaf Water Hyacinth (Eichhornia diversifolia)
Red Hygrophila (Alternanthera reinecki)
Hornwort (Ceratophyllum submersum)
Water Millfoil (Myriophyllum elatinoides)
Note that several of these are likely non-native and/or invasive.