Spływ kajakowy zaczyna się przy przystani kajakowej.

It is a fixed, very natural Polish expression for a kayak trip, especially a trip along a river.

A literal breakdown is:

• spływ = a float trip, river descent, or organized trip downstream
• kajakowy = kayak-related, from kajak
  • -owy

So spływ kajakowy is not just any activity involving a kayak; it usually suggests a proper river route or organized outing.

Because Polish often uses the verb zaczynać się / zacząć się to mean to begin.

Compare:

• zaczynać coś = to start something
• zaczynać się = to begin

So:

• Organizator zaczyna spływ. = The organizer starts the trip.
• Spływ zaczyna się... = The trip begins...

In this sentence, the trip is not starting something else; it is itself beginning, so zaczyna się is the right form.

Not really in the normal English sense.

In many Polish verbs, się is part of how the verb works, and you should learn the whole expression together:

• zaczynać = to start something
• zaczynać się = to begin

So although się historically has a reflexive meaning, here it is best understood as part of the verb phrase, not as a separately translated word like itself.

That is the normal placement.

In Polish, się is a short clitic word, and it usually does not stand at the very beginning of a sentence. It often comes after the verb:

• Spływ kajakowy zaczyna się...

Other word orders are possible in Polish, but zaczyna się is the most natural neutral order here.

Because the preposition przy requires the locative case.

The dictionary form is:

• przystań = landing, dock, marina, kayak station

After przy, it changes to locative singular:

• przy przystani = by/near the landing

So this is a case ending issue:

• nominative: przystań
• locative after przy: przystani

Here przy means something like by, near, or at.

So przy przystani kajakowej suggests the starting point is by the kayak landing/dock, or in its immediate area.

It does not necessarily mean physically on the dock. If you wanted to stress being directly at/on the landing itself, Polish might use a different preposition in some contexts, such as na.

Because Polish adjectives must agree with the noun they describe in gender, number, and case.

Here:

• spływ is masculine singular nominative, so: kajakowy
• przystani is feminine singular locative, so: kajakowej

So the adjective changes form to match the noun:

• spływ kajakowy
• przy przystani kajakowej

This is very normal in Polish and one of the biggest differences from English, where adjectives do not change.

No.

These are different words:

• przystań = a landing place, dock, marina, waterside station
• przystanek = a stop, such as a bus stop or tram stop

So przystań kajakowa is a place for kayaks, not a public transport stop.

Because Polish has no articles.

English needs words like a and the, but Polish does not. Whether something is definite or indefinite is usually understood from context.

So Spływ kajakowy zaczyna się przy przystani kajakowej could be understood as:

• The kayak trip starts by the kayak landing or
• A kayak trip starts by the kayak landing

Context tells you which is meant.

Yes. Polish word order is more flexible than English.

The original sentence is neutral and natural:

• Spływ kajakowy zaczyna się przy przystani kajakowej.

But you could also say:

• Przy przystani kajakowej zaczyna się spływ kajakowy.

That version puts more emphasis on the location.

So the meaning stays basically the same, but the emphasis changes.