Query m:n-Combination

Dear list,
I have to solve a simple Problem, explained below with some sample-Data.

A typical M:N-constellation, rivers in one table, communities in the
other table, m:n-join-informations (which river is running in which
community) in a third table.

Table rivers:
R_ID R_Name
1 river_1
2 river_2
3 river_3
4 river_4
5 river_5

Table communities :
C_ID C_Name
1 community_1
2 community_2
3 community_3
4 community_4
5 community_5

Join-table
mn_2_r_id mn_2_c_id
1 1
1 2
1 3
1 4
2 1
3 2
3 5
4 3
...

(in real database this relation is an gis-relation with thousands of
rivers and countries, related by spatial join, but the problem is the
same...)

I want to know, which river is running through communities 1,2,3 *and* 4?
You can see the solution by just looking at the data above (only
"river_1" is running through all these countries), but how to query this
by sql?

Thanks in advance
Ludwig