der Fluss
river
A large, natural flowing body of water that empties into a sea, a lake, or another river.
Die Donau ist der zweitlängste Fluss in Europa.
The Danube is the second-longest river in Europe.

river / flow
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river
A large, natural flowing body of water that empties into a sea, a lake, or another river.
Die Donau ist der zweitlängste Fluss in Europa.
The Danube is the second-longest river in Europe.

The noun 'der Fluss' (river/flow) is closely related to the verb 'fließen' (to flow). This connection helps you understand both the literal meaning (a body of water that flows) and the figurative meaning (a steady stream).
Das Wasser fließt langsam im Fluss.
The water flows slowly in the river.
In the figurative meaning, '-fluss' (flow) is often used as the second part of compound nouns to describe a continuous stream. Common examples are 'Redefluss' (flow of speech), 'Verkehrsfluss' (traffic flow), or 'Gedankenfluss' (flow of thoughts).
Nach der langen Pause kam der Verkehrsfluss nur langsam wieder in Gang.
After the long break, the traffic flow only slowly got going again.
The idiom 'im Fluss sein' (to be in flux) means that something is in a process of change and is not yet completed. It is often used for plans, projects, or situations.
Die Organisation des Festivals ist noch voll im Fluss.
The organization of the festival is still very much in flux.
development
'die Entwicklung' and 'der Fluss' can both have to do with change. 'Entwicklung' describes a process in which something changes or grows over a longer period of time. In this relation, 'Fluss' more often means a steady movement or a state in which nothing has been fixed yet, especially in the phrase 'im Fluss sein' (to be in flux). 'Entwicklung' sounds more precise when you are talking about concrete progress or changes. 'Im Fluss sein' works well when a plan, situation, or process is still open and in motion.

Unsere Pläne sind noch im Fluss, weil wir auf eine Antwort warten.
Our plans are still in flux because we are waiting for a reply.
Die Entwicklung des Kindes ist für die Ärztin sehr wichtig.
The child's development is very important to the doctor.