Show layers on the search map:
Rivers, HUC2 Boundaries
Showing the HUC4 Boundaries when you hover.
Maybe highlighting the HUC4 as you hover the pill, or possibly when you click the HUC4 number.
Users probably don't know what a HUC number is all about, so either highlighting on the map, or providing an info box, or adding titles to the columns, or all of the above.