We illustrate the fundamental importance of fluctuations in natural water flows to the long-term sustainability and productivity of riverine ecosystems and their riparian areas. Natural flows are characterized by temporal and spatial heterogeneity in the magnitude, frequency, duration, timing, rate of change and predictability of discharge. These characteristics, for a specific river or a collection of rivers within a defined region, shape species life histories over evolutionary (millennial) time scales as well as structure the ecological processes and productivity of aquatic and riparian communities. Extreme events-uncommon floods or droughts-are especially important in that they either reset or alter physical and chemical conditions underpinning the long-term development of biotic communities. We present the theoretical rationale for maintaining flow variability to sustain ecological communities and processes, and illustrate the importance of flow variability in two case studies-one from a semi-arid savanna river in South Africa and the other from a temperate rainforest river in North America. We then discuss the scientific challenges of determining the discharge patterns needed for environmental sustainability in a world where rivers, increasingly harnessed for human uses, are experiencing substantially altered flow characteristics relative to their natural states. = 65.4 ± 6.2%.
SUMMARY1. River valleys resemble dynamic mosaics, composed of patches which are natural, transient features of the land surface produced by the joint action of a river and successional processes over years to centuries. They simultaneously regulate and reflect the distribution of stream energy and exchanges of sediment, wood and particulate organic matter between riparian and aquatic environments. 2. We determined the structure, composition, dynamics and origin of seven patch types at the reach scale in the Queets River valley in the temperate coastal forests of the Olympic Mountains, Washington (U.S.A.). Patch types included: (1) primary and (2) secondary channels; (3) pioneer bars; (4) developing and (5) established floodplains; and (6) transitional and (7) mature fluvial terraces. 3. Lateral channel movements strongly shape patch distribution, structure and dynamics. The primary channel moved laterally 13 m year )1 , on average from 1939 to 2002, but was highly variable among locations and over time. Mean lateral movement rates ranged from 1 to 59 m year )1 and moving averages (2 km) ranged from 3 to 28 m year )1 throughout the valley. 4. Each patch type exhibited characteristic vegetation, soil and accumulations of large wood. Pioneer bars contained peak stem density (69 778 stems ha )1 ) and volume of large wood (289 m 3 ha )1 ). Mature fluvial terraces contained the highest mean stem (1739 m 3 ha )1 ) and canopy volume (158 587 m 3 ha )1 ). These patches also contained the most soil nitrogen (537 kg ha )1 ) and carbon (5972 kg ha )1 ). 5. Patch half-life (the time required for half of the existing patches to be eroded) ranged from 21 to 401 years among forested patch types. Erosion rates were highest in pioneer bars (2.3% year )1 ) and developing floodplains (3.3% year )1 ), compared with only 0.17% year )1 in mature fluvial terraces. New forests formed continually, as pioneering vegetation colonised 50% of the channel system within 18 years, often unsuccessfully. 6. In the Queets River, the structure, composition, and dynamics of the patchy riparian forest depends on the interplay between channel movements and biophysical feedbacks between large wood, living vegetation and geomorphic processes. The cycle of patch development perpetuates a shifting-mosaic of habitats within the river valley capable of supporting diverse biotic assemblages.
Large logs, important agents of biophysical heterogeneity in temperate floodplain rivers, have been virtually eliminated from modified systems. Our purpose was to quantify the sources and dynamics of large logs (> or = 1 m diameter) in the mainstem of a nearly pristine system: the Queets River, Washington, USA. Erosion of forests by the river supplies 0.40 logs x (100 m)(-1) x yr(-1) to the channel. Most (72%) are new logs entering the river for the first time as the river undercuts mature fluvial terraces dominated by large conifers. Retrospective airphoto analyses demonstrate that, over 63 years, the Queets River recruits 95% of new logs from a riparian corridor extending 265 m laterally on both banks, mostly through channel meandering. However, input rates are patchy, with 10% of the valley length supplying 38% of the new logs. As the river moves laterally, remnant logs are left on channel surfaces that later develop riparian forests and reenter the river when those forests erode. Remnant logs lying on the floodplain forest floor surface or buried in alluvium constitute 21% and 7% of the annual inputs from bank erosion, respectively. We estimate that 50% of logs deposited in the channel in a given year, including those underpinning logjams, are transported downriver within five years. Over the next 55 years, bank erosion reclaims an additional 23%, leaving 27% of the logs stable for > 60 years. Simulations indicate that recurrent transport is common, with half of the large conifers being deposited in > or = 3 locations and transported > or = 1.5 km prior to disintegrating. One in ten logs links distant reaches by occupying > or = 7 locations spanning > or = 12.0 km. Instream supplies are therefore a mixture of new and old logs from nearby and upstream forests, sustained by the recapture and transport of stockpiled remnant logs during periods when new inputs are low. We propose that patchy input rates and the periodic rearrangement of large logs are important drivers of temporal variation in river valley habitats, adding to the spatial complexity created by stable logs. These findings underscore the importance of extensive mature forests and connectivity in temperate floodplain rivers.
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