modifiers - What is the difference between an adjective before the noun and after the noun?

For a long time I'm having trouble understanding the difference between two kind of expressions like those below in terms of meaning, not grammar:

1. Excited people are looking forward to seeing this event.


2. People excited are looking forward to seeing this event.

EDITED TO USE CLEARER EXAMPLES:

3. "All navigable rivers are being patrolled."


4. "All rivers navigable are being patrolled."

As a native speaker, how do these expressions in bold sound to you? Is there any difference in meaning between examples #3 and #4? If so what is it, and why is this so?

Answer

The problem is that grammar is somewhat tied to meaning here. The position of an adjective in a sentence depends on its role.

When used attributively (to describe a noun), as stated in other comments and answers, the adjective comes before the noun:

All navigable rivers are being patrolled.

If you say:

All rivers that are navigable are being patrolled. (Others are not)

This can become:

All rivers navigable are being patrolled.

At first glance this doesn't really seem to change the meaning since:

rivers that are navigable = navigable rivers

---

Edit: But...

When an adjective comes after the noun it describes (like in the 3rd example), it functions as a postpositive modifier. Changing the position of the adjective (relative to the noun it describes) may bring a slight difference in the meaning of the sentence (the meaning of the word itself does not change!). When used postpositively an adjective connotes an ephemeral quality, one that is present at the moment, but doesn't always have to be. On the other hand, the adjectives used attributively may express either an ephemeral or a permanent characteristic, depending on the context. The difference between attributive and postpositive use of an adjective is explained in more detail in (the middle of) this post and in the comments.

---

Only some adjectives can be used both attributively and postpositively (while retaining the same word meaning), and these are the ones ending in -able and -ible (such as navigable). (But not even all of those - see later: responsible).

To cover another aspect (this is where grammar kicks in again): if an adjective is used predicatively (in a pattern: subject + verb + object + complement (here an adjective)) it would be in a sentence like this:

Signalisation on the banks made rivers navigable. (Or something like that, I'm not really an expert on rivers).

The upcoming event made people excited.

The meaning of some adjectives (when used as modifiers) changes depending on whether they are used attributively or postpositively. Some examples are: concerned, responsible, present etc. Neither navigable nor excited are among those. Here the meaning of the word itself changes and the difference can be determined by checking the dictionary definitions.