If you've ever spent time along a stream of any size, you may have wondered about the alternating patterns of flat versus tumbling water - what river scientists refer to as features. What's behind that? Why isn't it all one continuous feature? And if you've been faced with the challenge of crossing a stream, finding the appropriate stream feature can help you cross safely with minimal effort. Or, knowing where those pools get deepest, might influence where you cast your line in search of a trophy trout.
Natural streams have repeating features that maintain channel slope and stability. In a sinuous lower slope stream meandering across a valley floor – such as the East Branch in Keene Valley or the West Branch along River Road – riffles, runs, pools, and glides will appear in sequence. Higher slope streams tumbling down a hilly or mountainous grade follow a step-pool sequence – such as John's Brook in its higher elevations or Cascade Brook. In each case, these stream features interact with the bed, banks, and floodplain of a stream to create an ecosystem network that supports wildlife habitat, oxygenates water, and continuously rebuild stream form and function.
In most Ausable streams, the riffle is a bed feature made of larger cobble or boulders forming a single or short series of rib-like structures from bank to bank. The water depth is relatively shallow, and the water surface slope is steeper than the average slope of the channel. At low flows, water moves faster over riffles, which removes fine sediments and provides oxygen to the stream. Riffles enter and exit meanders and control the stream bed elevation. They provide a good starting point if you are looking for a place to cross a stream. Runs will follow riffles, though are deeper with their surface water appearing less turbulent. Pools are located on the outside bends of meanders between riffles or occur behind large protruding boulders in the channel or along the banks. Pools have a near-flat water surface (the product of a very low slope) and are much deeper than the stream's average depth. At low flows, sediment and cobble is deposited in pools and are scoured away from riffles. At high flows, however, the pool is scoured and the bed materials deposit on the riffle. This occurs because a force applied to the stream bed, called shear stress, increases with depth and slope. Depth and slope increase rapidly over the pools during large storms, increasing shear stress and causing scour. A glide is the section of the stream coming out of the pool, from deeper to shallower as it approaches the next riffle. River scientists often refer to the pool tail or the tailwater in reference to the tail-like surface water formation that marks the transition from glide to riffle.
Stream channels, corridors, and floodplains form a valuable ecosystem network. They transport water and sediment and provide habitat for many aquatic organisms including fish, amphibians, aquatic insects, mollusks and plants. Features such as riffles and pools, runs and glides, form a diversity of aquatic habitats and are essential to the biological and water quality functions that streams provide. Small invertebrates, macrobenthic organisms, cling to rocks and coarse substrates in riffle areas, filtering food from the flowing water and thriving in the oxygen-rich water. Fish species utilize meander pool areas, taking advantage of the cover provided for protection and feeding and benefitting from the cooler water temperatures in these deeper waters. Even within a single meander pool, there are aquatic organisms that prefer to live at varying water depths and locations within the pool, reinforcing the natural diversity and varying biological functions that healthy stream systems provide. (Harman et. al. 2012)
Taking the time to identify and measure stream features in healthy reaches of a river can help us assess and address problems in sections that are experiencing erosion, widening, or areas that seem less resilient. But understanding the sequence of features in your local stream will enhance your appreciation of its unique nature, experience its gradual changes, and help you enjoy its intricate and practical beauty.
To learn even more: "A Function-Based Framework for Stream Assessments and Restoration Projects" May 2012 by Will Harman, Rich Starr et. al., is searchable on the web. The framework provides a comprehensive approach for understanding streams and assessing and restoring stream form, function, and health.