Streams come in all shapes and sizes, from tiny trickles to lazy rivers. As far as science defines it, even the mighty Mississippi River is a stream.
You probably don’t have anything as big as the Mississippi on your land, but you might have its beginnings, or the beginnings of some other river. Just because your streams are small, doesn’t mean they’re worthless. Small streams flow into larger streams, so what happens in those first few drops affects everyone and everything down the line. Even if your streams could never support a minnow, they still matter for wildlife.
But what makes a stream better or worse for animals? Generally speaking, if you want to attract creatures to your streams, focus on achieving the 3 C's: clean, cold, and complex.
This might sound like an obvious one, but it’s the most important. The best streams for wildlife have clean, clear water. They’re free of chemical pollutants like pesticides, low in nutrients, and low in fine soil particles like silt and clay. These pollutants can lead to algae blooms that can kill fish, and the fine soil can smother fish eggs.
There are many ways to keep your streams clean, and most have nothing to do with the streams themselves. Instead, they’re all about the land. Because water flows downhill, every inch of your property eventually drains into some body of water somewhere. That means what you do on your land has the potential to impact local water supply and the animals that use it.
Like us land critters, animals that live in water need oxygen to breathe. But while we have plenty of oxygen around in us the air, water-bound creatures aren’t so lucky. Yes, water is part oxygen, but the oxygen in the water itself isn’t available for animals to use. Aquatic creatures rely on oxygen gas that gets mixed into the water and dissolved. Appropriately, this oxygen is called “dissolved oxygen.”
Think of dissolved oxygen the way you might sweeten tea by adding sugar. You pour in the sugar, stir, and the sugar dissolves. Dissolved oxygen works the same way. As water flows in a stream, the churning current picks up oxygen from the air and mixes it into the water.
The catch is that the concentration of dissolved oxygen in water is really low. There’s about 20 percent oxygen in the air we breathe, but water only has about 1 percent dissolved oxygen. That’s not a lot for animals to use.
What does all this have to do with temperature? Cold water can hold more dissolved oxygen than warm water. That means as a stream warms, it loses its dissolved oxygen, and animals have a harder time breathing. To have the greatest variety of creatures in your stream, then, colder is better.
Just as different land animals need different habitats, aquatic life also needs complexity. Some critters need fast, shallow water with a lot of dissolved oxygen. Others can’t hang on in a current and prefer deeper, slow spots. In the same way that having a mix of older and younger woods will increase wildlife use of your land, making your streams more complex will too.
The classic structure for healthy streams is the “riffle-pool-run” trio. Maintaining this structure in your streams – or restoring it if it’s been lost – is essential if you want the most wildlife.
Riffles are shallow, rocky-bottomed areas. Because of their uneven beds, riffles have disturbed surfaces and extensive mixing of air and water. As a result, riffles are important for adding dissolved oxygen to the stream. They’re also important for fish eggs. The big spaces between larger stones keep the eggs safe from the current while exposing them to large amounts of dissolved oxygen to help the baby fish develop.
The opposite of riffles are pool – the slow-flowing, deeper areas of streams. They’re important for animals that can’t handle faster flows, in particular young fish.
In between riffles and pools are runs, areas of moderate current and depth. Runs have few obstructions or changes in the direction, so water flows smoothly through them.
Less healthy streams consist largely of runs, with straight flow and little variation. By contrast, healthier streams will be more complex. They’ll have fewer runs and many deeper pools with corresponding shallow riffles.
Reprinted with permission from Attracting Wildlife to Your Backyard (2018), by Josh VanBrakle and published by Skyhorse Publishing.
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