Arwen saves Frodo from the Nazgûl by invoking a flood. This site was used to film the shallow water scenes. The wider shots were filmed in Skippers Canyon.
Click the image to toggle between the original and a colour-graded version that matches the actual film scene.
GPS coordinates: -44.9346, 168.8325
The link opens in Google Maps (desktop) or in the Google Maps App (Android).