The Bridge Effect

An example of a bad culvert. Notice the

water fall effect that prevents fish passage.

It's also too small and can't handle high

water flows.

A bridge is not a simple structure. To the eye they may seem simple, just a series of intersecting pieces that allow you to span a river, creek, canyon, or bay. But all the pieces must work together to support the weight of the entire bridge and all its traffic on just a few supports. Of course some designs are simpler than others. A small bridge crossing a 10 foot creek is a child’s plaything compared to a bridge spanning the Columbia River. Bridges can be made of many things. Cement, metal, and wood being some of the more common materials, and they’re often combined depending on the need.

As much fun as it would be to write a blog focusing on the types bridges I feel that’s a little outside the areas I should focus on. Instead I want to focus on bridges and their environmental impacts, both good and bad. I want to keep a very loose definition of bridge open for discussion, so I’m going to be looking at bridges and bridge-like structures. I’m not going to be bashing bridges either.

The first area I’d like to look at include the effects on aquatic ecosystems. Large aquatic ecosystems, such as a large river, bay, sound, etc. may be less impacted by a bridge. That’s not to say there’s no impact, especially during construction, but once completed the water and any organisms present can generally navigate around the pylons with little impediment.

Small aquatic ecosystems are easier to negatively impact. Things like creeks and small rivers. Places where an improper bridge can severely limit the flow of that ecosystem. I’m going to include culverts here. I consider them bridge-like.

The same site replaced with a bridge. Notice the open water.
Fish and other wildlife can freely travel. 

Culverts can severely reduce the health of aquatic ecosystems if they’re too small or improperly installed. They can separate fish populations or prevent the migration of salmon to spawning habitat. They can also limit the movement of aquatic insects and amphibians. Culverts that can’t handle high water flows can lead to washouts of surrounding vegetation and soil further negatively impacting the water health. Culverts without natural surface bottoms can disrupt wildlife by confusing them as they move around. Luckily these have simple solutions. Replace them with larger culverts that properly connect the stream, or better yet, remove the culverts entirely and install an open span bridge. This allows for the proper meandering of the stream and lessens the impact on wildlife.

Bridges spanning rivers can also be poorly built and while they might not affect the aquatic ecosystem as much they can impact the terrestrial ecosystem. Many animals move throughout their day near water. If they’re walking along the bank and suddenly run into a large cement bridge where do they go? Maybe they can swim, so they just hop in the river and go around. Maybe they turn around and go back. Or maybe they want to keep going. They wander up onto the road, possibly endangering themselves and human drivers.

Another possible option. A much larger culvert. There 

is no longer a waterfall and it can handle high water flow.

However, bridges can be built specifically for the protection of animals that may want to move across a highway or highly congested area. Often these are termed wildlife crossings, and include a variety of structures including overpasses, underpasses, and culverts (all bridges or bridge-like structures). They are built solely to allow animals to move without having to cross in front of traffic.

Thankfully, most negative impacts of bridges are considered when new bridges are being built. All aspects of the ecosystem are considered, aquatic, terrestrial, and even, aerial (some birds run into suspension bridge lines). Many new bridges look to mitigate and even reverse the negative impacts of their forebears resulting in bridges that accommodate human needs to travel, but also try to preserve natural ecosystems.