The Ashley River / Rakahuri is a Canterbury river draining a large catchment that backs onto the Southern Alps, making it highly responsive to rainfall — flows can rise quickly when rain falls in the headwaters. The most paddled section is Ashley Gorge, running from Middle Bridge to the Campground, a stretch that delivers continuous grade III rapids over approximately 4 kilometres once the technical water begins. Above this, the Lees Valley to Middle Bridge section offers easier water in the II to II+ range at lower flows.

Flow levels strongly determine the character and appropriate skill level for the gorge. At 5–10 cumecs, the gorge runs at II/III and is the preferred range for packrafters. Between 20 and 50 cumecs, the rapids grade up to a solid III and represent the preferred window for most kayakers. At 60 cumecs and above, the river becomes expert-only territory, grading III+, with boulder drops flushing out and powerful holes developing. At these higher flows, some eddies become difficult or impossible to exit, and certain features become unscoutable and unportageable. The standard advice is to put on after rain has stopped and flows are dropping rather than rising.

The gorge demands strong group rescue skills at kayaking flows. Swimmers can lose their boats quickly, as boats flush through rapidly and are rarely recovered. Throw ropes are only effective if the rescuer is positioned upstream. Most search-and-rescue callouts on this river involve paddlers separated from their craft. One notable hazard is a large rock on the first significant rapid, located on a left turn after a long straight section — scouting from an eddy is recommended rather than running it blind. The long rapid leading to Big Eddy carries pinning potential in midstream, and creative rescue techniques may be required if a pin occurs there.

At its lower end, the Ashley River / Rakahuri flows into a tidal lagoon estuary — the Ashley-Rakahuri / Saltwater Creek Estuary (Te Akaaka) — a semi-enclosed embayment where fresh and salt water mix. The estuary supports diverse ecological communities and is a recognised area of ecological significance. A public viewing platform accessible from the Ashley River end of Waikuku Beach provides a vantage point over this coastal environment.