Gweek Quay slipway

The Helford is a ria (a drowned river valley) rather than a true river, and Atlantic tides push right up to Gweek at high water springs. The main river runs for around 9.2 km, with a tidal area of 568 hectares and 44.3 km of shoreline at extreme high water. How much water is here depends entirely on the tide: above Tremayne Quay the channel dries to mud and a narrow marked passage, so getting afloat and getting back is tied to a suitable tide rather than a fixed time of day. Downstream of Gweek the river opens into seven creeks in turn — Ponsontuel, Mawgan, Polpenwith, Polwheveral, Frenchman's, Port Navas and Gillan — with the drying channel below Gweek marked by port and starboard buoys kept up by voluntary contribution.

Merthen Wood, a Site of Special Scientific Interest, runs for nearly two kilometres of ancient, near-pure sessile oak along the river's north shore between Gweek and Helford; there is no public land access into the wood, but it is visible from the water. The wider estuary carries several conservation designations, including a Voluntary Marine Conservation Area since 1987, part of the Fal and Helford Special Area of Conservation, and the Lower Fal and Helford Intertidal SSSI, protecting features that include eelgrass beds and one of the UK's largest maerl beds, along with intertidal mud and sand flats that are important feeding grounds for birds. The main eelgrass beds and the voluntary no-anchoring zone that protects them sit off Grebe and Durgan, near the river mouth well downstream of Gweek. The estuary is also a private tidal fishery owned by the Duchy of Cornwall and a designated bass nursery area, with oyster beds occupying parts of the river above Port Navas.