The storm was short and intense but the sky stayed gray long after the rain stopped. Walking outside, I saw little waterfalls cascading down the previously dry gullies that dotted the landscape. The stream – which hours before had been a grassy depression – was filling with water, turning what was a dry creek bed into a full, free-flowing stream.
This was last July at my in-laws in eastern Colorado. In this semi-arid region (dry, but not quite a desert) all but the largest rivers are empty most of the time, waiting for the next storm to send water tumbling down their channels. Many people who have visited the arid west are familiar with these flash floods, or have seen evidence of their destructive power on the news. But how common are these streams and what are the lives like of the creatures who inhabit them?
Temporary waterway is the general term for rivers that flow during certain times, and are dry the rest. These come in two flavors: ephemeral streams, which only flow after storms, and intermittent streams, which are fed by groundwater but can dry up when groundwater levels drop. While these types of rivers are common in drier areas, they are also present in many wetter regions. In fact, the majority of small streams in the upper parts of watersheds are ephemeral or intermittent. Even though they may not always be flowing, these temporary streams form important connections to larger downstream rivers and provide habitat for a variety of animals.
How do fish and other water-dependent animals survive in streams that go dry for long periods of time? They are often able to find refuge in scattered pools near springs or under the shade of a cottonwood. These fish bide their time, waiting for the next storm that will fill the channel, creating a temporary fish highway1. One such pool is right in the middle of my in-laws property and provides year-round habitat for frogs, birds, and fish (check out the picture on the top of this page).
We have altered many temporary waterways, changing the amount and frequency of flows they carry. I study a stream near Denver named Big Dry Creek. Several wastewater treatment plants discharge water continuously into the channel, changing Big Dry Creek from an ephemeral to perennial (constantly flowing) stream. Similar changes in the Phoenix area have converted many dry washes to permanently flowing streams, completely changing the ecology of these rivers. On the other end of the spectrum, groundwater pumping in eastern Colorado and Nebraska is drying up intermittent rivers, leaving fewer pools essential for fish survival.
Many people, if asked to define the word river, would say it is a channel with flowing water. But what about these streams that are not constantly flowing? Do they cease to exist when they run dry? This is an important question with legal ramifications2. The science suggests that no, these temporary waterways do not stop being rivers just because they dry up for part of the year. In fact, these streams are always an important part of river networks.
Back at my in-laws, I was stomping around the freshly-filled stream. Part of why I find rivers so interesting is that they are constantly changing. And there is no greater example of this than these often waterless rivers.