How to show river in heightmap in mountains?

I have troubles with making world. I want to make a game with terrain from real place (really big place). I use https://heightmap.skydark.pl for heighmap because terrain.party doesn’t work already for a year. So, my place is in mountains and there is some rivers which I need in my game. But in heightmap the river and nearest flat place is just black. This is because mountains are high and only 255 levels of b/w is not enough for my 1 kilometer mountain. What do I do? Place coordinate is longitude:107.28543460907275 and latitude:49.046330856596825. Doing this all by sculping will take years. It could be done by adding colors to heightmap but it didn’t work like that.