Introduction | People | Fish | Habitat Activities | Index | Feedback | New Activities Weirs A weir consists of interlinked panels that stretch across the whole river. The type of weir shown in this photograph consists of floating panels that are anchored to the river bottom on the upstream side. This allows water to flow through, but stops the upstream passage of fish. The center of the weir has a chute with a gated holding pen. As salmon swim into the pen, biologists count the fish and identify the species, then release them to continue swimming upstream to spawn. Some of these fish are also measured and the sex is determined. Back to the activity Text and graphics by USFWS staff Last modified 18 January, 2006