Streams - Culverts as Barriers
Just as roads and urban centers can be barriers for mammals on the move, barriers to fish movement can exist within stream systems. Many fish, like Arctic
Grayling, Walleye, Pike and Bull Trout, will swim many kilometres upstream to reach their spawning areas in the tributaries to larger rivers. These migrations can be blocked in a number of ways. Sometimes in small streams, low water levels make upstream movement difficult, or fallen timber might cause an obstruction that fish cannot get over or around. Structures in streams like hydroelectric dams, beaver dams and culverts may also interfere with fish migrations.
Culverts are the most commonly installed structure when there is a need to cross a small stream. They are easy to install and are relatively inexpensive. A park may install a culvert where a stream flows year-round or for part of the year, to ensure that vehicles and people can safely cross the stream to access campsites or other parts of the park. A forest company may install a culvert if a stream flows across the road they use to transport logs.
Culverts are designed to funnel water through a tunnel-like structure so that stream flow is not interrupted. How, then, does this block fish migration?
When fish migrate, they follow along the bottom, or bed of the stream. When the water drops out of the end of the culvert, like a small waterfall, we call this an "overhanging" culvert. Streams are dynamic, changing with environmental conditions. In contrast, culverts are rigid structures placed into these dynamic systems. So why not build a bridge? Bridges cost more, and are intended to be permanent structures, while culverts are often temporary. But temporary can mean it still remains in place for several years.
Flash floods and stormy situations can result in more debris in the stream than usual. If the culvert is not as wide as the stream, what will happen to debris moving downstream when it reaches the culvert? Is this debris any different than what a beaver may create when building its dam? Is one type of stream blockage more acceptable than another type of stream blockage?
Improperly installed culverts can block upstream fish movements. Fish cannot enter overhanging culverts or are unable to swim through long culverts because of the high velocity of water flowing through them. But, when people take the time to install the culvert properly, and to keep it free of debris, we can have our stream crossing, and fish can have an obstacle-free stream. Culverts do not have to be barriers!
Reprinted from Stream Connections with permission of Alberta