procedural terrain generating

any good tutorials about procedural terrain generating /prog/? I am currently particularly interested how rivers can be generating, how do the affect the terrain.

>>6
like this?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define _(i,j) d[(i)+(w+2)*(j)]
#define __(i) for(int i=0;i<w;i++)

int main(){
  int w, arr[]={620757024, 621552898, 620766464, 623650064,
                622077186, 622601474, 624174360, 624698660};
  scanf("%d",&w);
  short *dat=(short*)malloc((w+2)*(w+2)*2), *d = dat+w+1;
  srand(time(0));
  __(i)__(j)_(i,j) = (j&&j+1-w?0:5) | (i&&i+1-w?0:10) |  rand()%4&(w-1-j?w-1-i?15:14:13)
                   | (j?i?15:11:7)  & (_(i-1,j)&1 | _(i,j-1)&2)<<2;
  __(i)__(j)_(i,j) = *(((short*)arr)+_(i,j)),*(int*)(d+i*(w+2)-2) = 655373;
  fwrite(&(d[-1]=-257),2,w*(w+2)-1,fopen("o.txt", "wb"));
  return 0;
}