To help clear up some confusion:
A rivulet or streamlet is the smallest form of naturally moving water.
The other forms, in order of size, are brooks, streams, creeks, and rivers, which are the biggest.
The streams which lead into a river are called tributaries.
A canal is a man-made waterway. A defensive canal around a structure is a moat.
A phytotelma (lit. "plant pond") is the smallest form of naturally occurring still water. They form inside of the cavities of some plants.
A puddle is larger than that, followed by a pool.
A pond is larger than both of them, and a lake is the largest. In the UK, they are sometimes called lochs.
An oxbow lake or billabong is a body of water cut off from the river it was formally part of.
A spring is usually where a waterway starts, bubbling up from underground.
A delta is where many streams flow into an estuary before they reach an ocean or sea. They are usually triangle shaped.
However, that's only freshwater.
Larger than a lake is a sea, and many seas make up an ocean, but are often largely enclosed by land.
A channel connects two bodies of water between two landmasses. When one is narrow, it is often called a strait.
A sound is an area of sea usually surrounded by mountains or hills.
A gulf is a part of the ocean which has overtaken part of the land. It is usually almost completely enclosed by land. One with a wider mouth may be called a bay. A smaller bay or gulf is called a cove.
An inlet is a recess along the coast. A deep inlet with steep sides is known as a fjord.
An oasis is a body of water located in a dry or desert land.
A lagoon is a body of water cut off from a larger body of water. Sometimes a coral reef encircles it, creating an atoll.
An archipelago is a group of islands. If an archipelago encloses a reef or lagoon, it is called an atoll.
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(Disclaimer: Unfortunately, these are just general descriptions that give an idea of how different geographic features are differentiated. These terms are often used for locations which do not fit the above definitions.)