Riwaka was released in the 1990s by New Zealand's hop breeding program, derived from Saaz breeding stock in the same general lineage as Motueka, and named after the small town of Riwaka in the Nelson growing region. Despite that noble-hop ancestry, Riwaka's aroma reads nothing like its restrained Czech parent — it's one of the most intensely aromatic hops New Zealand has produced, packed with passion fruit, grapefruit and lime at a comparatively low alpha acid.
At 4.5-6.5% alpha acid, Riwaka is used almost entirely for aroma, and its oil content is potent enough that even a modest dry-hop weight delivers a pronounced tropical-citrus character — brewers new to the variety sometimes over-dose it expecting a low-alpha hop to need a heavier hand, then find the resulting beer's aroma considerably more intense than planned. Riwaka is grown in relatively small quantities compared with New Zealand's larger commercial varieties, which keeps it in comparatively tight supply and pushes its price a step above the country's higher-volume exports — and the IBU calculator will confirm how little of Riwaka's bitterness actually matters next to its aroma.
Nelson Sauvin is the closer substitute for sheer aromatic intensity, though it carries a wine-and-gooseberry character rather than Riwaka's more straightforward passion-fruit-and-citrus profile. Motueka offers the more direct genetic comparison from the same Saaz-derived breeding line. Hazy IPA and New England IPA both use Riwaka, generally in smaller doses than its low alpha might otherwise suggest.