Indeed! The Cataracts are similar to reefs, except they actually have a flow direction, though it's hard to see in the static screenshot. This is a picture of Cataracts (flowing southwest to southwest) that I used to represent the Sarn Gebir on the Anduin River in my map.
Cataracts (similar to the Gold scenario resource) won't show up in normal games because in the modinfo for the DLC scenarios, it'll only activate if that specific scenario is being played.
As for the first part of OP's question, I've already dug up the cataract files from the Nubia scenario as a mod, since I'm also using them in a custom map. Beware though, the actual placement of Cataracts is very non-user-friendly. Some caveats:
- Cataracts MUST have a flow direction manually assigned to them. Unlike all other features, Cataracts will appear as a red exclamation point if no direction is assigned. In WorldBuilder, click on "Plot", select the tile with the Cataract and choose a "Feature Direction".
- Because the Feature was designed only for the Gifts of the Nile scenario, the directions of water flow only exist for the 5 configurations that exist in the Gifts of the Nile scenario (e.g. it is impossible to have a Cataract flow west or southeast because the scenario map doesn't have them, and thus Firaxis didn't design the respective graphics for them.)
- The Cataract animation actually includes two directions, an inflow direction and outflow direction. Sometimes both are the same, but not always. Both inflow and outflow directions correspond (arbitrarily) to one WorldBuilder direction (in bold below).
- No Direction: not used (will appear as Red Exclamation Point if selected)
- Northeast: NE inflow to E outflow
- East: NW (both inflow and outflow)
- Southeast: SW (both inflow and outflow)
- Southwest: N inflow to NE outflow
- West: NE (both inflow and outflow)
- Northwest: not used (will appear as Red Exclamation Point if selected)
- You cannot have Tundra or Snow tiles of any type adjacent to the Cataracts, or else it'll cause the graphic to glitch.
- Activating my mod will make you unable to start the Gifts of the Nile scenario. You'll have to disable this mod before loading that scenario.
Since you are aiming to recreate a grander version of the Gifts of the Nile scenario map, some of the caveats won't apply to you. If you are recreating the existing parts of the Gifts of the Nile scenario map 1:1, the directions you want for the six Cataracts (from south to north) are Northeast, East, Southeast, Southwest, West, Southwest
I considered releasing this as a Mod on the Steam Workshop, but honestly, (especially with how janky their use is) I think you and I are the only two people that would ever use Cataracts in a custom map. Because of that, I think it would be more convenient for the people playing our maps to just have the Cataract feature built in to the particular map mod itself instead of as a separate dependency.
Cataract Features Mod download (It's an external link because I can't seem to upload it directly to the forum).
Good luck, and feel to ask if you have any questions! Happy mapping!