Seal Slough is a tidal channel at the northern edge of Foster City that supports a permanent colony of harbour seals. At low tide, the seals haul out onto the mudflats in numbers — sometimes dozens at a time — and can be watched from the footpath along the channel edge. This is one of the few places in the Bay Area where wild harbour seals are consistently visible without a boat trip.
The slough is part of a larger wetland system and is good for birdwatching. Great blue herons, great egrets, and various shorebirds are common throughout the year. Access is on foot from the residential streets at the north end of the city; the walk from the main park takes about 20 minutes.