Mark Beardsley, M.S., EcoMetrics, LLC and CRA Board Member
Volume 22, Number 3, Fall 2011
In the course of studying meadow streams in the montane and subalpine zones of the Rocky Mountain Region, I have had considerable difficulty finding good reference reaches. On nearly all of the undisturbed meadows with stream orders 1-3 (and occasionally 4), channel morphology and function is controlled much more by beaver activity than by any other factor. Single-thread stream channels are often the exception rather than the rule; with multiple-channel, ponded beaver dam complexes a more common occurrence. Beaver-controlled stream systems are natural and stable. They are often the historical pristine condition, and are commonly a stable endpoint of natural channel/riparian evolution. They possess a unique suite of ecological functions that are quite different from other defined stream channels, and should therefore be recognized as a legitimate, naturally occurring channel type in stream classification schemes. I propose the definition of a new stream type to recognize channels that have form dictated primarily by beaver activity.
Much recent progress in stream science and restoration technology can be attributed to the development of useful and accurate systems for stream classification. At the present time, the Rosgen classification scheme sees the most widespread use, and is the basis for widely used methods of stream assessment, restoration, and research. Categorizing stream channel types based on numerous morphological variables, the scheme adequately covers most stable stream channels found in nature. Unfortunately, the system currently has no effective means for dealing with smaller streams that take their form primarily from the dams, steps, ponds and braids that are maintained by beavers, and this leads to confusion among scientists and practitioners. Recognizing the beaver-dominated stream as a legitimate, unique channel type within the Rosgen classification system is an effective way to remedy this situation by accounting for this common natural channel type. A logical name for this class is DB. ‘D’ implies the multiple-channel nature of these reaches, while the subscript ‘B’ may be considered a descriptor for ‘beaver’ or ‘biotic’ control as opposed the existing stream categories of D for braided systems or DA for anastomosed.
Morphological description and stability of the DB stream type
Stable beaver-dominated stream types occur in valley types II-VIII. These streams typically consist of multiple channels, and are highly variable in terms of entrenchment, width-depth ratio, and sinuosity. These are dynamic systems with morphology controlled by beaver dams that generally maintain a severe pool-drop profile, and a braided pattern. Dam construction, maintenance, and periodic failures drive sediment transport and storage and these are the primary processes that control stream form. The DB stream type is unique in this way because of the overwhelming influence of biotic factors (beavers and a large woody plant community) that drive channel morphology. Nevertheless, the DB stream type is unexplained by any other existing category in the current Rosgen Stream classification, and it is unique enough to earn its own class.
There may be some reluctance to consider beaver-dominated, DB streams a natural, stable stream channel type. When viewed in the short term, we tend to think of beaver dam complexes as net sediment sinks with accumulation and aggradation a common feature. This may be so, but in the long term these systems may be just as effective at transporting sediment as other stream classes. While individual ponds do tend to accumulate sediment by deposition, other areas in the system rapidly release sediment when dams burst or new channels headcut from overland flood flows. Sediment tends to move through these systems in pulses, as beaver dams fail and frequent avulsions occur, both of which are common processes in mature beaver dam complexes. Transport efficiency depends on a balance between the net effects of these competing, dynamic processes. In reality, DB reaches may be stable, aggrading, or degrading over the long term, just like any other stream type.
Ecological function and general habitat characteristics of the DB stream type
Beaver-dominated stream systems have unique habitat characteristics that differ significantly from their single-channel counterparts. Generally, the aerial extent of aquatic habitat is much greater in DB reaches compared to single-channel streams that would occur in similar valleys. Wetted width values at both bankfull and low flow tend to be extremely high, and it is not uncommon for a beaver-dammed stream to take up nearly the full width of its valley. In other words, the amount of available aquatic habitat, per unit valley length, is very high in these systems. Typically, most of the aquatic habitat area in DB reaches would classify as pools. Riffles and other facet types are relatively rare since, as in other step-pool systems, most of the stream gradient is lost in steep drops, but the reaches usually do maintain some riffle habitat.
With the high frequency of deep ponds and plunge pools, the amounts of pool cover, velocity refuge, and overwinter habitat for fish are especially high. Beaver pond reaches typically support high levels of fish biomass. Pond pools with very low velocity also tend to accumulate fine sediments over large areas, so substrate composition is effectively altered, and this has significant effects on macroinvertebrate communities. These streams offer habitat for lentic, fine substrate benthic macroinvertebrates in addition to the species that thrive in the scour areas of plunge pools and riffles. In short, DB reaches tend to provide plentiful, diverse aquatic habitat that can support greater biomass and diversity of aquatic life.
Water temperature and chemistry effects of DB reaches are complex. The most significant factor is the spreading of water that occurs on these reaches in the many ponds and overland flows. This would tend to maintain high water tables and promote hyporheic exchange. Increased ponding and spreading would tend to increase diurnal water temperature fluctuation on these reaches, but these effects may be buffered by the increased groundwater exchange and shading. Chemical effects are also complex. Frequently saturated soils would tend to promote anaerobic conditions, low redox potential, and the slow decomposition of organic material that is typical of wetlands. It has been suggested that beaver dam complexes are important for the absorption and removal of toxic minerals from water and sediment.
The hydrologic effects of DB reaches such as groundwater recharge, flood retention, and energy dissipation are well-understood for beaver complexes. From a watershed perspective, the DB stream type functions in buffering the hydrograph of its drainage and making it less flashy. Peak flows and floods are lower and longer-lived. Equally important, DB streams maintain base flows for a longer portion of the season as stored water is released slowly. The maintenance of base flow is especially important in arid areas where aquatic biota may be limited by lack of water during summer or winter.
From a terrestrial habitat point of view, DB reaches are also ecologically productive and diverse. By spreading water and maintaining high water table, these streams promote widespread hydrophilic plant life. The episodic nature of sediment capture and release as dams are constructed and eventually fail constantly creates patches of bare ground at the floodplain elevation for colonization. This dynamic process provides a mechanism for maintaining high species diversity in the riparian plant community as new opportunities for species establishment are continually being produced. It also maintains a great variety of habitat patches that differ wildly in saturation levels and other abiotic conditions.
Significance and the need for recognizing the DB stream type
The Rosgen stream classification scheme is by far the most practical and widespread method of stream classification in use. Adding the DB stream type class to this system provides a more realistic depiction of the variety of stream forms in nature. At the same time, it gives a practical means to more accurately describe, assess, and restore ecological health of streams.
DB systems are functionally different, and often functionally more beneficial, than their alternative single-channel counterparts. Additionally, the DB stream type may be the highest probability stable state in natural or disturbed systems. From a practical standpoint, including the DB class allows us the opportunity to consider a beaver system as the potentially stable condition, and to use reference DB reaches as benchmarks for assessment and divergence analysis. Likewise, DB reference reaches could serve as analogs for restoration design and legitimate targets for riparian restoration or protection.
The beaver-controlled system is a common, natural stream type that is not effectively defined by any of the existing classes in the Rosgen stream classification scheme. To remedy this shortcoming, an additional class (DB) is proposed as the multiple-channel beaver-dominated stream type. In addition to making the classification a better representation of the variability of naturally stable stream types, recognizing this class will improve our ability to make accurate assessments of stream ecological condition and to design more efficient and natural ecological restoration projects.