Complexity refers to the physical heterogeneity, or diversity of habitats in river ecosystems. Complexity is influenced by: the number of channels (single thread vs. multi-thread) channel bank irregularities, channel cross section, channel planform, sinuosity, type and abundance of riparian and aquatic vegetation, large woody debris (LWD), and geomorphic units (e.g., pools, riffles, backwaters). In general a stream that has been straightened, incised, lacks riparian vegetation and is dominated by plane-bed features would not be described as complex. By contrast a stream of comparable size, characterized by high sinuosity, multiple channels, abundant pools, riffles and backwaters, high substrate variability, significant LWD and a diverse and abundant riparian area would be considered complex.
Physical complexity is a hallmark of diverse and resilient river ecosystems. The diversity and abundance of habitats available for aquatic and terrestrial organisms increases with physical complexity. Irregular banks, sinuous planform, LWD, pools, riffles and backwaters and abundant riparian vegetation help to create and maintain habitats used by a diverse biological community that has different habitat requirements. Additionally, single species may utilize different habitats during different times of year and at different life stages.
Ecosystems that are characterized by high levels of physical heterogeneity are also more likely to resist and recover from disturbance events (resilience). Within river corridors, riparian vegetation, channel-floodplain connectivity, LWD, and diverse geomorphic units (pools, riffles, backwaters) mitigate extreme conditions such as flooding (by promoting dispersal onto the floodplain, and providing flow refugia) and drought (deep pools provide thermal refugia and riparian vegetation provides shade).
Complexity increases sediment and nutrient retention by altering hydraulics and creating areas where sediment and nutrients can be stored. Rivers characterized by high complexity have high hydraulic variability. In other words, there are areas of high flow velocity and low flow velocity, areas of flow convergence and divergence. This variability influences the formation of a diverse set of geomorphic features including pools, riffles, and backwaters. It also influences patterns of sediment erosion and deposition. Features such as LWD and beaver dams decrease flow velocity and promote the deposition of sediment, organic matter and other nutrients, and increase the residence time of these materials which increases the likelihood of uptake by microogranisms or their stabilization by aquatic and riparian vegetation. In the case of beaver dams, sediment may be stored for centuries in beaver meadows. Reduced sediment and nutrient delivery to downstream reaches improves water quality, which benefits downstream river ecosystems and humans (drinking water, recreation, aesthetics.)
Complexity in river systems is created and maintained by a number of different factors and their interactions, including: flow regime, riparian vegetation, lateral connectivity, longitudinal connectivity, large woody debris and beaver activity. Alterations to any of the natural processes that contribute to complexity/physical heterogeneity can affect a river ecosystem. The relationship between the magnitude of alteration, and the magnitude of decrease in complexity is a challenging to assess and is based in part on the geomorphic setting. For example, a highly confined steep bedrock river will be less susceptible to changes in flow than a laterally unconfined river with an extensive floodplain. Different rivers, in different settings will exhibit different degrees of complexity. Therefore understanding the unique controls operating along specific reaches, valley bottoms, and basins is critical to understanding the specific processes responsible for creating and maintaining complexity in a given reach.
Beaver are most well known for their dam building activities. Beaver dams alter the timing and magnitude of flow by creating ponds that temporarily store water, force water overbank into riparian areas, and promote groundwater recharge. When compared to reaches without beaver dams, reaches with beaver dams experience lower peak discharges and higher baseflows. Beaver ponds are also capable of capturing sediment and nutrients. As water approaching a beaver dam slows, sediment is deposited, reducing downstream delivery of sediment. By increasing access to water resources, beaver dams help promote a healthy riparian corridor. In addition by altering patterns of erosion and deposition, upstream and downstream of dams, beaver dams create surfaces that can be colonized by riparian vegetation.
Unlike man-made dams, beaver dams are temporary. Beaver may abandon their dams, dams may breach (partial removal of dam material) during high flows, or they may blow out (full removal). Beaver are also susceptible to predation from numerous species and trapping by humans. It is important to recognize that complexity, and the processes that maintain it, changes through time. In other words, the partial breaching, or full scale blow out of a beaver dam does not necessarily have negative implications for complexity. In fact, the breaching of a dam creates many new surfaces of variable elevation and substrate characteristics that can promote riparian colonization, and the creation of new instream aquatic habitat such as pools and riffles.
There is extensive literature on how practices such as dam building, flow alteration, channelization, deforestation, conversion of forests to agriculture or urban areas, historic land uses such as mining and logging and extirpation of beaver have reduced riverine complexity. (For links to many of these, see Resources and Links).
Complexity in many river systems has been reduced due to direct human alteration (e.g., channel straightening, wood removal and flow alteration) as well as changes in land use and land management (conversion to agriculture, urbanization and grazing). Many streams and rivers now exist in low complexity states and lack the ‘tools’ that can lead to increased complexity. In rivers where the channel has been straightened and wood has been removed high flows may no longer reach the floodplain, and a lack of LWD decreases flow velocity variability that creates and maintains diverse geomorphic features such as pools and riffles. Channel straightening and LWD removal can also contribute to a feedback wherein the channel straightening leads to and increased tendency to incise, resulting in further loss of channel floodplain connectivity which can lead to decreased water table elevations and reduced likelihood of riparian recruitment. Where channel banks have been armored to prevent erosion, recruitment of LWD from riparian areas is no longer possible. In areas where the flow regime has been altered by dam operations or diversions, peak flows (and flow variability) are often reduced allowing the encroachment of vegetation that can result in reduced lateral adjustment capacity. Daily fluctuations for power demand and high flows from dams that do not correspond with the life histories of riparian and aquatic organisms prevent the recruitment and survival of those organisms, often accentuating the initial negative effects.
Restoring complexity, also commonly referred to as habitat heterogeneity, is a common goal in river restoration efforts whose objective is to improve ecosystem health and resilience. Restoring complexity requires restoring the conditions that allow rivers to maintain complexity (outlined above). This means both changing the initial conditions (e.g., adding LWD or beaver dams to a stream, restoring a natural flow regime) and ensuring that pressures on other parts of the system are reduced in order to promote the feedbacks that ensure complexity can be maintained. For example, introducing LWD and/or beaver dams increases hydraulic variability that creates and maintains patterns of erosion and deposition that result in diverse geomorphic units. They affect the hydrologic regime by locally raising water tables and increasing the extent and frequency of overbank flows (increased floodplain connectivity). In settings where there is the potential for future LWD recruitment and growth of a riparian area, ‘kickstarting’ by LWD or beaver dam additions may allow the processes that encourage their maintenance to persist. By contrast, a one-time addition of LWD at a local scale, without allowing for the growth of a riparian community capable of growing LWD is unlikely to be sustainable in the long term.