All search results carry implicit as well as explicit keywords. The trick is to realise those implicit words, and work them into your SEO efforts in order to rank better for the explicit words.
For instance, search for ‘River amazon’ and it’s clear Google pushes the keyword ‘facts’ in the SERPs. This is an obvious and simplistic example, but illustrates that with a bit of digging, you can find hidden gems – second or third keywords to think about adding to your metas.