You will probably spend many hours in dug-out canoes or boats, according to the type of trip that you will have chosen.
The fauna found in rivers and on their banks is generally easier to spot than the one living inside the rainforest, since the field of vision is much wider.

But you have to look! This can seem evident, but I still remember a sweet lady who, after having slept for nine-tenth of the time during a long dug-out trip, complained about its monotony and the paucity of visible animal life…

What can you reasonably expect to see?
Many water birds, of course, some of them spectacular such as the scarlet ibis.

But you will also be able to admire certain birds as they fly across the river, the slow flight of the macaws, or the comical one of the toucans. Having very short wings, their flight consists of a series of successive flappings interrupted by a short dive.

The famous freshwater dolphin: there exists two species:

The Sotalia (sotalia fluviatilis), a small dolphin which is not specifically a freshwater dolphin but which swims far into the large rivers such as the Amazon or the Orinoco.
The true freshwater dolphin (inia geoffrensis). It is a primitive dolphin with a very elongated nose and minute eyes. As all dolphins do, it finds its position and hunts by sound echo. It only dwells in freshwater.

To distinguish one from the other, look at their back. The sotalia has a crescent-shaped back fin whereas the inia's is not well marked and has the shape of a triangle with a large base.

The largest of freshwater otters, the giant otter of Brazil (pteronura brasiliensis) lives in the Amazon and Orinoco zone.
It is a magnificent 2 meter long animal, counting its tail.
It captures large fish, I saw some eating 10 kg heavy catfish. It is relatively curious and in places where it is not hunted for its fur, it can sometimes come quite close to dug-out canoes.
I have been told of cases of "association" between otters and dolphins to bring fish in. I have in fact seen dolphins and pteronura swim very close to each other but maybe they were simply over the same fish shoal.

The galapago turtle (podocnemis sp) can be frequently seen in certain places. You will see them better during the dry season, if there is much sun, they gather on beaches and rocks.
They can sometimes be seen comically standing in unstable equilibrium on branches nearly at water level. The large arran turtle (podocnemis expansa) can weigh as much as 100 kg but it is becoming rare because it is pitilessly hunted.

The crododilians

The most common is the spectacled cayman (caiman crocodilus).
In quiet places, it too likes to bask in the sun but most of the time, you will have to be content with seeing its bright orange eyes, by night, in your lamp beam. It rarely reaches 2 meters in length.

Caiman Crocodilius

Many reptiles, like galapago and arran turtles and certain crocodilians deposit their eggs on the river beaches during the dry season. They dig a deep and well-hidden hole but, for those who have wide-awake eyes, their tracks on the sand betrays them.

Usually, when they have found a nest, Amerindians take only part of the eggs. It is unfortunately not the case with everyone. This plundering is one of the main causes of rarefaction of certain species. The spectacles cayman lays its eggs inside the forest, behind river banks. Perhaps that is why it is so abundant…

One of the best ways to see monkeys if from the river, after heavy night rains, especially if the sun shines.
The large spider-monkeys (ateles sp), for example, like to dry themselves at the top of the large trees.

The river banks are haunted by tapirs, capybaras and pacas.

And of course, there are the very many fish species (over 2400 in South America): :
Certain fish are enormous, like the famous arapaima or pirarucu (arapaima gigas)
Some sheat-fish (brachiplatystoma sp) that weigh over 200 kg! Others are superb. A large part of the aquarium fish sold in Europe originates from the Amazonian rivers, such as the magnificent "discus" (symphysodon sp).

Unfortunately, you will only have an infinitesimal chance of seeing the Manatee or sea-cow (trichechus sp), this curious aquatic herbivore. This enormous animal which vaguely looks like a seal has been decimated everywhere. Furthermore, it is extremely discreet, spending its days feeding on water hyacinths. In the Orinoco Delta, these plants grow in such quantities due to the disappearance of the sea-cows that they block out navigation over wide expanses.

Finally, to go from the animal to the plant kingdom, those who love orchids will be able to easily observe through their spyglasses certain large light-loving species, such as the cattleya orchid.

To end with rivers, let us speak of "black rivers" and "brown rivers".

The "brown rivers" have a muddy colour for they are full of silt and rotting plants. It is the case of the Amazon river and the Orinoco, for example.

The "black rivers" are tea or coffee colour of a more or less deep hue. They drain white and sandy soil originating from the very ancient mountain range of the Guyanas. These soils are very poor in mineral content. The forest grows literally on the platform ground for there is nearly no fertile layer.

The coffee colour comes from substances issued by the decomposition of leaves : phenols and tannins. It may be that these elements represent a protection against grass-eaters, I say "may" for it is not known for certain.

One of the hypotheses trying to explain the presence of a strong concentration of "defensive" chemicals in plants living on this type of soil is that the latter is so poor that it is more "advantageous" for the plants to "invest" in protective chemicals. Trees grow more slowly but are better protected.

I have somewhat insisted on this technical point because you will often hear that there are few mosquitoes near black rivers whereas they abound near brown rivers.
Personally, I am not sure. I have been devoured on the edges of certain black rivers and I have set camp in swimming suit on the edge of certain brown rivers.

In general, in the primary rainforest, there are few mosquitoes, even on river edges. During the dry season, bivouacs are very pleasant. There are some mosquito rushes when rains come but nothing compared to what can be encountered in Northern Europe during the summer!
From time to time, generally after the first rains, there are invasions of horseflies. This painful situation usually lasts a few days. They are diurnal and disappear at sundown.

In fact, paradoxically, it is sometimes necessary to take cover in the forest in order to avoid being stung! The savannah zones are sometimes infested by minute blood-sucking flies. In certain places, they are so numerous that one has to protect one's face with a small mosquito-net. These insects are not found in woodland.