摘要:

AbstractA 25 m long transect was positioned along the spray gradient of a waterfall (880 m a.s.l.) in the Aurland River, western Norway. The vegetational composition in contiguous 1 m × 1 m quadrats was recorded in 1968 before regulation of the river, and again 20 years later. Spray precipitation, temperature, and humidity were also measured. The 92 per cent reduction in mean annual discharge of the river has resulted in a 98‐100 per cent reduction in spray precipitation along the transect. In general, there has been a great decrease in hygrophilous bryophytes and alpine vascular plants, and an increase in lichens and drought‐tolerant bryophytes. At the end of the transect furthest from the waterfall, which previously received less than 1200 mm of spray precipitation per year, there has only been minor floristic change. Similarly, in the nearest 5 quadrats, where spray precipitation is still substantial, there has been little change in the vegetation. The greatest change occurred in the middle of the transect which formerly received substantial amounts of spray, but is now almost

ISSN:0886-9375    

DOI:10.1002/rrr.3450060302

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTwentymile Creek, a sand‐bed stream draining a 450 km2catchment in northeast Mississippi, was channelized prior to 1910, in 1938, and in 1966. Straightening and enlargement in 1966 was followed by channel instability—rapid bed degradation (2‐4 m) and cross‐section enlargement by 1.4 to 2.7 times. Grade control structures (GCS) (weirs with stoneprotected stilling basins) and various types of streambank protection were constructed along the channel in the early 1980s to restore stability. Other investigators have suggested that habitat recovery in incised, channelized streams is facilitated by construction of GCS because they create stable scour holes and promote natural formation of a low‐flow channel flanked by vegetated berms. Effects of restabilization of Twentymile Creek on aquatic habitats were assessed in four ways. The fraction of the bank line covered by woody vegetation was mapped from aerial photographs taken in 1981 and 1985; physical habitat (depth, velocity, substrate, and cover) and fishes were sampled at base flow; and the existence and size of a low‐flow channel was ascertained from cross‐section surveys taken in 1980 and 1989. Woody vegetation, physical aquatic habitat, and fishes were also sampled from Mubby‐Chiwapa Creek, a similar‐sized unstable channel with no GCS. Physical habitat variables and fishes were sampled concurrently at five stations on Twentymile Creek, and four stations on Mubby‐Chiwapa. Four of the five Twentymile stations were either above or below a GCS. Bank‐line woody vegetation cover increased 8 per cent between 1981 and 1985 along Twentymile Creek but was stable along Mubby‐Chiwapa. Reaches above and below GCS were deeper with slower current velocities than elsewhere. Mean Shannon diversity indices based on physical data were similar for both streams, but were 58 per cent higher for stations immediately above and below GCS than for other stations. Since construction of the GCS and bank protection measures, longitudinal berms have formed within the enlarged Twentymile Creek channel, creating a low‐flow channel. Low‐flow channel capacity was equivalent to a mean daily discharge equalled or exceeded 30 per cent of the time, and was considerably lower than the effective discharge. Differences in aquatic habitat diversity among the stations sampled were primarily due to the scour holes below the GCS and the low‐flow channel. Thirty‐nine fish species were collected from Twentymile Creek, but only 22 from Mubby‐Chiwapa. Fourteen species were collected exclusively at GCS. Principal component analyses of the abundance of the eight numerically dominant fish species indicated similar faunas at most stations, but Twentymile Creek GCS stations were faunistically distinct. Abundance of several of the numerically dominant species was positively influenced by greater depths and lower velocities found near Twentymile GCS. The mean fish diversity index for Twentymile Creek was 29 per cent higher than for Mubby‐Chiwapa, and fish diversity was positively correlated with

ISSN:0886-9375    

DOI:10.1002/rrr.3450060303

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMacroinvertebrates were sampled in seven microhabitats (submerged woody debris, reeds, sedges, inundated grass, floating aquatic vegetation, lignum, and the unvegetated littoral) at thirteen sites representing six macrohabitats (single temporary and permanent ox‐bow lakes (billabongs), fast and slow‐flowing anabranches, backwaters, and the main channel) of the River Murray, and Australian lowland river. Sites were sampled in spring 1988, shortly after floodwaters had receded.Most of the 95 taxa collected were aquatic insects. Detritivores were most abundant in all macrohabitats except the temporary billabong where predators predominated. Scrapers were only abundant in the permanent billabong. The temporary billabong harboured the most species and individuals whereas slowly‐flowing anabranches contained the fewest species and individuals. At the microhabitat level, most taxa and individuals occurred in stands of aquatic vegetation. The unvegetated littoral zone was the most depauperate microhabitat.Multivariate analyses illustrated the distinctiveness of the faunal assemblage found in the temporary billabong. Subsequent analyses of the permanently inundated macrohabitats indicated gradients related to current velocity and the extent to which the sites were continuous with the main river. Current velocity apparently determined assemblage composition at the macrohabitat scale whereas the structural complexity of submerged vegetation operated at the microhabitat scale.The relatively large number of taxa collected from this area emphasizes the importance of a range of macrohabitats and microhabitats to faunal diversity in a floodplain ecosystem. Although there was little faunal overlap between billabongs and the main river, billabongs probably serve as refuges for many lentic taxa that rely upon regular inundation to survive. Surveys of floodplain rivers for management purposes must include samples from aquatic habitats adjacent to the main channel because the fauna of the floodplain is potentially most threatened by regulation and alteration of the flooding r

ISSN:0886-9375    

DOI:10.1002/rrr.3450060304

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAs a part of a study investigating the hydraulic effects of large woody debris (LWD) in lowland rivers, a series of small‐scale experiments were conducted in a rectangular glass‐walled recirculating flume. These experiments were undertaken to determine the order of magnitude of the increase in flood levels caused by LWD at different positions within a channel cross‐section. Position variables that were considered in these experiments were height above bed, angle to flow direction, and separation distance in the direction of flow. This study was undertaken to quantify the hydraulic benefits (primarily reduced flood levels) gained by the removal of LWD from lowland rivers, which is a common practice in several countries. From an integrated river management perspective it is necessary to weigh any hydraulic benefits of LWD removal up against the environmental costs of loss of faunal habitat, and possible geomorphic instability.The results of these experiments indicate that the levels of LWD commonly occurring in the lowland rivers of southeastern Australia seldom cause any significant effect on flood levels. However, where LWD occur at channel constrictions, or where unusually high densities of LWD are present, the effect on flood levels will be signif

ISSN:0886-9375    

DOI:10.1002/rrr.3450060305

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMacroinvertebrate assemblages downstream from the Canning Dam, on the Canning River, Western Australia, were sampled to assess the impact of long‐term impoundment and the role of a major tributary in community recovery.Kick samples and associated physical measurements were taken from riffle zones in three reaches in March and July 1989. The lower reach was located immediately downstream of the confluence with Stinton Creek, the first major tributary below the dam, with the middle and upper reaches positioned between the tributary and the dam.Reduced concentrations of dissolved oxygen in the upper and middle reaches in March, compared to the lower reach were attributed to significantly higher levels of organics, high summer water temperatures, and a proposed increase in microbial activity. The build‐up of organics was probably related to reduced flushing, as a result of impoundment. Stinton Creek increased the discharge of the Canning River below the confluence, particularly in winter, which presumably prevented the build‐up of organics in the lower reach.Significant differences in the aquatic macroinvertebrate fauna between reaches were detected. A total of 68 taxa was recorded from the lower reach, 88 per cent of which were also present in the middle and upper reaches. However, the middle and upper reaches contained a greater number of taxa (112 and 90 respectively), approximately 50 per cent of which were not recorded from the lower reach. The additional taxa were more typical of lowland rivers or lentic (standing water) systems, suggesting that physical conditions in the middle and upper reaches were more like a lowland river than an upland stream.More collectors and shredders occurred in the upper and middle reaches, associated with the accumulation of particulate organic matter. Ordination and classification procedures based on macroinvertebrate assemblages clearly separated samples from the upper and middle reaches from the lower reach. There was also a distinct seasonal separation.These observations support a hypothesis that while the reduced flow below Canning Dam had an impact on the macroinvertebrate fauna, confluence with a major tributary (Stinton Creek) allowed recovery of the macroinvertebrate community, through the tributary acting as a source of increased discharge. The implications for the management of impounded rivers in southwestern Australia are disc

ISSN:0886-9375    

DOI:10.1002/rrr.3450060306

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe importance of substrate type and water depth was investigated in a regulated reach of a large, navigable river in the U.S.A. Benthic grab samples were collected in June and August, 1989, along two shore‐to‐shore transects in the lower Tennessee River, approximately 1.5 km downstream from Kentucky Lock and Dam. Diversity, averaged over all samples, was not significantly different from June to August. River depth of our samples, which ranged from 3.1 m to 6.5 m, significantly influenced macroinvertebrate density but had no detectable effect on the total number of taxa collected. Densities of flatworms, Asiatic clams, snails, isopods, and mayflies increased significantly with increasing depth, whereas the abundance of native mussels (Unionidae) and caddisflies decreased with depth. Substrate type (sand through cobble) influenced both the total number of taxa and the total number of organisms collected. Sample areas dominated by cobble contained significantly fewer individuals than areas composed predominantly of gravel substrate. Asiatic clams, the single most abundant species collected, were consistently more numerous on intermediate substrate sizes. In contrast, densities of native mussels, snails, and mayflies were not correlated with bottom type. Sample areas included relatively little mud and sand, resulting in relatively low numbers of oligochaetes and midges in our collecti

ISSN:0886-9375    

DOI:10.1002/rrr.3450060307

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0886-9375    

DOI:10.1002/rrr.3450060309

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0886-9375    

DOI:10.1002/rrr.3450060310

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0886-9375    

DOI:10.1002/rrr.3450060301

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY