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