Tides are the oceans’ slosh, long-period waves caused by the tug of the moon and the sun, affected by the Earth’s rotation and the moon’s orbit. Incoming and outgoing tides spread estuarine silt across the intertidal zone, sands near the low tide line, and mud nearer the high tide line. At the ebb, tideﬂats stretch in bare, shining expanses for miles in big shallow estuaries such as Willapa Bay, Washington. A tidal ﬂat’s ﬂatness depends on its fauna of clams, oysters, shrimp, crabs, and worms, and its ﬂora of eelgrass, cordgrass, and algae. Atlantic cordgrass (Spartina alterniﬂora) valuably anchors tideﬂats in the East, where it is indigenous; as an introduced alien in the West, it makes dense meadows hostile to oysters and shorebirds. Through the tideﬂats run sinuous, brackish sloughs and channels. Tidal inlets push seawater into outﬂow, stirring salinity into freshwater as it goes. The Columbia River is tidal more than a hundred miles upstream, beyond the city of Portland to Bonneville Dam.