The water breaks through because upstream – far enough up that there isn’t any stream – there’s a drip, drip, drip.
Enough drips to puddle, to pool and start to trickle and then to run: a rivulet, a stream to cool your feet in.
Further on* lakes, rivers, waterfalls, floods, torrents to burst banks and blow your socks off.
All it takes is gravity, time, and enough drips.
*in no particular hydrological order