It happened gradually, and through the second generation. George H. Reifel first leased his father's land to the British Columbia Waterfowl Society in the 1960s, which is the moment the ground stopped being agricultural in purpose even while it kept its dikes and ditches. The federal migratory bird sanctuary was established in 1967. The 1972 agreement made the arrangement permanent. It is a modest-sounding sequence of transactions, but it produced something unusual: a wetland in the most heavily farmed, most heavily diked part of the Lower Mainland that nobody can drain, develop or repurpose.

The sanctuary covers 648 hectares, of which roughly 300 are the part visitors walk - managed wetlands, marshes, and the dikes that hold them in shape. That word, managed, matters. The water levels here are controlled, raised and lowered on a schedule that suits the birds rather than the weather, so that the shallow pools sit at the right depth for a probing bill in the weeks that probing bills arrive. Trails run out along the dikes to bird blinds and lookouts, there is a gift shop, and the gates are open from nine in the morning until four in the afternoon, Tuesday through Sunday, with reservations now required for every visit. It is not wilderness. It is a machine for producing habitat, and it runs well.

Sandhill cranes nest here - tall, grey, prehistoric-looking birds whose bugling call carries across the entire marsh and startles first-time visitors into stopping mid-path. Bald eagles hold the cottonwoods. Mallards and spotted towhees go about their year-round business. Then there are the ones just passing through, for whom this place is a fuel stop on a journey of continental length: lesser snow geese in white drifts, greater and lesser yellowlegs picking along the margins, long-billed dowitchers working the mud in synchronised sewing-machine motion, and western sandpipers in flocks that turn light and dark together as they bank. Over 250 species have been recorded within the sanctuary.

The sanctuary has a season, and it is not summer. When the lesser snow geese come down the coast, the delta changes character entirely. They arrive in white drifts that settle onto the marsh and the neighbouring fields, and the noise they make is less a call than a weather system - a continuous, layered yelping audible long before you reach the dike. Then something spooks them, and several thousand birds lift at once, turning the middle distance into a blizzard moving sideways. It lasts a few seconds. They come back down. Nothing else in the Lower Mainland is quite like standing in the middle of it on a cold, flat, grey December morning with the Fraser running brown a few hundred metres away.

Understand what a site of Hemispheric Importance means and the rest of it follows. Shorebirds migrating the Pacific coast cannot fly the whole distance; they need places to land, and the number of places is finite and shrinking. The Fraser estuary is one of the essential ones - a soft, muddy, nutrient-loaded gap in a coastline that is otherwise rock. Lose it and the loss registers thousands of kilometres away, in nesting grounds in the Arctic and wintering grounds in South America. Which returns you to the two conditions the Reifel family attached in 1972, and to how much can turn on the wording of a single agreement about a farm.