There really isn't any way to re-create this, except luck or a seed filter. What happens is a river generates inside a swamp biome. The depth of the river is subtracted from the swamp floor, and the color of river biome makes a cool pattern. Seed, F3, and coords are in the attachments.
MC-221825
River biomes intersect and collide with swamp biome
Resolved
Duplicate
1
Thanks for your report. There was already a report for this (MC-3710), which I've linked to this issue; that issue was previously resolved as works as intended, but I've reopened it (it's probably still WAI but Mojang should confirm that).
Note that you're using 1.16.4 with Optifine, but selected 21w13a as the version. We only support the latest version on this tracker (1.16.5 or 21w13a), but since 1.16.4 and 1.16.5 are essentially the same, I've added 1.16.5 to the other ticket. We also don't support modded versions; in general, please test without any mods before making a report. Although Optifine does break many things, it seems unlikely to me that an issue that has been around this long is fixed in vanilla but re-broken by optifine, so I've just assumed that optifine isn't at fault here (hopefully I'm not wrong).