When an antique table is “restored” we know what that means – it’s been cleaned up and repaired to provide the same function it did when it was originally built. But determining what a “restored” stream is – and can be – is much less clear.
Pristine streams follow a path of “dynamic-equilibrium”, which means that while they are constantly changing their shape, form and hydraulic properties, there is an envelope within which the planform, cross-sectional and longitudinal dimensions and geometry remain stable. In this way, pristine streams balance flows and sediment transport in a manner that provides the most efficient conveyance of channel-forming discharges (~1.5 year storm as documented by Johnson, 1995 many others) while allowing large, infrequent flood events to access and inundate nearby riparian areas where flows are slowed, peaks are attenuated, and sediment is deposited. Macrobenthic invertebrates (stream critters) and other aquatic biota thrive in this environment of predictability, which leads to robust populations of biota as well as large biodiversity ranges.
But disturbances, such as quick alterations of the landscape (land clearing for agriculture in colonial times, urbanization in more recent times) throws this equilibrium out of whack, thereby altering the geomorphic properties of the stream until a new equilibrium is found. When trying to find this new equilibrium, the stream will become over-widened and much deeper (incised and entrenched in fluvial geomorphologic terms) thereby not enabling flood events to access floodplains, thereby leading to more and more in-channel erosion. Stream ecosystems begin to fail as species sensitive to wide ranges of hydraulic stressors drops and overall populations are reduced. The loss of trees near the stream accelerates, which further exacerbates the channel-widening associated with flashy urban flows in headwater streams.
Since the early 1990’s, efforts have been made to “restore” streams in urban areas that often focus on balancing the needs for geomorphic stability with constraints associated with urban landscapes. Banks are laid back to allow vegetation to become established and channel beds are raised to encourage flood flows to become re-attached to their floodplains. In-stream features using logs, rocks, or woody debris are installed to help the stream retain riffle-pool sequencing associated with healthy headwater streams (of moderate slopes). These efforts definitely encourage stability and improve biotic integrity of the stream corridor, but if flows within the watershed are not property addressed, there may be persistent problems of stability even after “restoration” is completed. Holistic care must be taken to address watershed needs while stream work is done, but this double-whammy of environment improvement is not often done leading to failed stream restoration projects – thereby giving the field a black eye.
First of all, we shouldn’t kid ourselves – it’s impossible to literally “restore” a stream back to its original form and function – namely because the changed landscape requires that streams perform different functions now. We can certainly attempt to stabilize stream and bring back critical aquatic biota, but urban streams can only bounce back so far. As a colleague once said, flood flows that wipe out streams in pristine conditions happen infrequently enough to allow the system to become re-stabilized. In this way, flood events act like forest fires, which help to replenish the forest canopy. However, when erosive events occur with an increasing amount of frequency (as is the case in urbanizing streams), the system never gets a chance to fully recover, and thereby permanently impairs the resiliency of the stream overall. In other words, the stream is always on fire – kind of like the Cuyahoga River in 1969 – but we just can’t see it as obviously as that.