Welcome back to our series on river ecology and conservation. This is a follow-up article to one published recently about headwater river ecology & conservation. If you haven’t read that yet, hop over there first! Otherwise, keep swimming downstream to learn more.
Rivers are an incredible and prevalent part of our landscape. Across North America, people live within an average of 2.2 miles from a freshwater body, including lakes and rivers of any size. While headwaters are more abundant, it’s often the mid-reaches of a river that folks personally connect with.
Mid-reach rivers describe 4th-6th order streams and rivers. Following the general assumptions laid out in the River Continuum Concept, a foundational stream ecology theory that describes rivers and their watersheds as a connected biophysical system from headwaters to the sea, these medium-sized rivers have a larger watershed area and greater discharge than headwaters. Unifying physical features of these rivers include:
With less shading from riparian areas and more exposure to sunlight, water chemistry shifts as well. Mid-reaches have warmer temperatures and lower dissolved oxygen as stream order increases.
With more sunlight, we can often find diverse algae and rooted aquatic plants in these rivers as well. These factors favor photosynthesis over respiration to form the basis of the food web.
Ask any angler, and she will tell you how important macroinvertebrates are to river food webs. In mid-reach rivers, aquatic invertebrates including
collect fine particulate organic matter for energy.
These tiny organic particles are made available by the upstream shredders,which are macroinvertebrates that tear up and consume leaves, twigs, and other coarse organic matter. In addition to the collectors, grazers such as snails represent the majority of primary consumers in midreaches.
Secondary consumers include macroinvertebrate predators such as dragonflies (Order Odonata, my personal favorite) and small minnows.
Biodiversity of fish tends to increase with stream order. Depending on the location of the river, there could be suckers, darters, catfish, sunfish, bluegill, and bass. These fish support numerous wildlife.
Healthy rivers with functioning food webs and good water quality provide habitat and resources for numerous kinds of wildlife. For example, hundreds of thousands of Sandhill Cranes rely on the braided channels of the Platte River in Nebraska during winter migration for safe roosting at night.
North American river otters, once thought to be extirpated from western North Carolina, now range year round from the mountains to the coast. Research out of Georgia suggests that protecting a variety of riparian areas with various elevations and canopy structure can support bat species richness.
Given the integrated nature of rivers from headwaters to the sea, rivers tend to function best when their flow is unobstructed. Unfortunately, many mid-reach rivers are impacted by obsolete run-of-rivers dams that no longer serve their original purpose.
A run-of-river dam is a structure built across a stream that allows water to flow over its top. These dams were most often used to divert water into canals for purposes like running mills, generating electricity, or irrigation.
However, dams can cause significant impacts on the environment by blocking the movement of water, sediment, and aquatic organisms. This can lead to negative changes to the characteristics of the stream and habitat.
Removing obsolete run-of-river dams is one way Unique Places to Save works to restore mid-reach rivers. Dam removals projects, like the Hoosier Dam Removal, provide benefits to water quality, fish passage, and wildlife.
The importance and benefits of protecting mid-reach rivers goes well beyond wildlife habitat. Mid-reach rivers provide many of the same benefits as headwaters in terms of flood protection, water quality, and aquifer recharging.
Their food webs and water chemistry heavily influence the health of larger rivers and estuaries. Thus, these rivers play a critical role in the water cycle as water moves from the atmosphere to the land and finally to the ocean.
These medium-sized rivers provide something else very important to conservation: accessibility to nature. These are often the rivers of our childhood memories where we learned to fish or canoe, took a float trip with friends, or went bird watching along a trail. In my case, I fondly remember learning to ice skate on a frozen pond alongside the Monocacy River.
At Unique Places to Save, we are dedicated to protecting aquatic resources because we know these streams, rivers, and wetlands not only shape our environment, but also enrich our lives. You can see our work to improve public river access at the dam in Ramseur, NC and other projects we have completed. Please consider supporting us to achieve more restoration and conservation projects across our country!