Robimy przenoskę, tym bardziej że w bystrzu leży przeszkoda.

Here robić przenoskę is a set phrase used in water-sports contexts, especially kayaking or canoeing. It means to make a portage: to get out and carry the boat or equipment around a dangerous or impassable section of river.

So Robimy przenoskę means something like We’re portaging or We’re carrying the boat around.

Because przenoskę is the accusative singular form of przenoska.

The verb robić usually takes a direct object in the accusative:

• robić przenoskę = to do/make a portage
• robić obiad = to make dinner
• robić zdjęcie = to take a photo

Since przenoska is a feminine noun ending in -a, its accusative singular changes to -ę:

• nominative: przenoska
• accusative: przenoskę

In Polish, the present tense can often describe something happening right now or a current decision. Here robimy most naturally means:

• we’re making a portage
• we’re going to portage
• we do a portage here

In context, it sounds like a practical decision being made on the spot: We’ll portage / We’re portaging.

Also, robić is an imperfective verb, so robimy presents the action as a process or ongoing action, not as a single completed event.

Tym bardziej że means especially since, all the more so because, or particularly because.

It introduces an extra reason that strengthens what was just said.

So the logic is:

• We’re doing a portage
• especially since / all the more because there is an obstacle in the rapid

This phrase is a little stronger and more nuanced than a simple because.

All of them can introduce a reason, but they are not identical.

• bo = because
  Very common, neutral, everyday.
• ponieważ = because / since
  More formal or written.
• tym bardziej że = especially since / all the more because
  Adds the idea that this reason makes the previous statement even more justified.

Compare:

• Robimy przenoskę, bo w bystrzu leży przeszkoda.
  We’re portaging because there’s an obstacle in the rapid.
• Robimy przenoskę, tym bardziej że w bystrzu leży przeszkoda.
  We’re portaging, especially since there’s an obstacle in the rapid.

The second one suggests there may already be other reasons, and this is an additional strong one.

Bystrzu is the locative singular form of bystrze.

Bystrze is a noun meaning something like:

• rapid
• fast current
• rough, swift part of a river

After the preposition w meaning in, Polish often uses the locative case:

• w domu = in the house
• w lesie = in the forest
• w bystrzu = in the rapid / in the fast current

The -u ending may feel unusual, but some neuter nouns form the locative this way. So:

• nominative: bystrze
• locative: w bystrzu

Here it is singular.

That can be confusing, because in English we often say rapids, which looks plural. But Polish can describe that kind of river feature with a singular noun:

• bystrze = a rapid / a stretch of swift current

So w bystrzu means in the rapid or in the swift water, not in the rapids as a grammatical plural.

Leży literally means lies. In Polish, it is often used for something physically lying somewhere.

So:

• w bystrzu leży przeszkoda = there is an obstacle lying in the rapid
• more naturally in English: there’s an obstacle in the rapid

Using leży gives a more concrete physical image than jest. It suggests that the obstacle is actually located there in the water, perhaps a rock, log, or some other blockage.

If you said jest przeszkoda, it would still be understandable, but leży sounds more vivid and natural for a physical object lying in a place.

Przeszkoda is in the nominative singular.

That is because it is the grammatical subject of leży:

• przeszkoda leży = the obstacle lies

Even though the word order is reversed:

• w bystrzu leży przeszkoda

the subject is still przeszkoda.

Both are possible, but they have slightly different emphasis.

• Przeszkoda leży w bystrzu
  More straightforward: The obstacle lies in the rapid.
• W bystrzu leży przeszkoda
  Puts the location first: In the rapid, there is an obstacle.

Polish word order is more flexible than English word order. Here, starting with w bystrzu helps present the location first, almost like setting the scene.

Because tym bardziej że introduces a subordinate clause giving a reason. In Polish, such clauses are normally separated by a comma.

So:

• Robimy przenoskę, tym bardziej że w bystrzu leży przeszkoda.

That comma is standard and expected.

It sounds fairly natural, but it is also somewhat specialized because of the vocabulary:

• przenoska is strongly associated with kayaking/canoeing/river travel
• bystrze is also tied to river language

A Polish speaker would understand the grammar easily, but the sentence clearly belongs to an outdoor or paddling context.

Yes, if you wanted less specialized language, you could say something more descriptive, for example:

• Musimy przenieść kajak lądem. = We have to carry the kayak over land.
• Omijamy ten odcinek pieszo. = We’re going around this section on foot.

But in the proper river-sports context, robić przenoskę is the natural compact expression.