Stream Morphology

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Impacts of an intense rainstorm on Whangapoua forest landscape and streams.

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This report decribes studies on the effects of the storm on 3-4 March 1995 on Whangapoua Forest’s landscape, peak streamflows, stream channels and instream habitat and invertebrate biota.

Characteristics and Geomorphic Effect of Wood in New Zealand's Native Forest Streams

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We surveyed the amount and geomorphic role of wood in 18 pristine native forest streams (channel width: 3–6 m) throughout New Zealand, and quantified the characteristics associated with piece stability and geomorphic effect. Wood piece numbers (18–66 per 100 m) and volumes (85–470 m3 ha–1) were similar to or greater than found in many streams throughout the world. Forest type and geographic location
had no discernable influence on wood abundance at a particular site, possibly due to the confounding influences of local features (e.g., tree fall regime) and methodology (‘snap-shot’ survey of a dynamic system). Half the pieces that were geomorphically active had moved, suggesting that stable pieces did not necessarily dominate geomorphic activity. Tree ferns were an important contributor to wood abundance in many of the streams studied.

Using morphological adjustments to appraise sediment flux

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Morphological change in gravelly river channels and active slope environments can be used to derive lower-bound estimates of sediment flux between successive dates on which the morphology of channels or slopes was measured. This paper provides examples of such morphological budgets derived from analysis of digital elevation models built from ground survey data acquired from three sites in the upper Motueka River, and the Tarndale Fan and Gully system in the upper Waipaoa catchment. We demonstrate the utility of such an approach in providing information on processes operating in channel and slope environments at the same time as estimating volumes of sediment flux.

The Pakuratahi Land Use Study

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Researchers compared pastoral farming with forestry over 12 years and found that a forest produces less sediment, uses slightly more water, reduces soil erosion, has a more positive effect on stream environments, and makes no real difference to water quality. Begun in 1993, the land use study was a response to public concern about the environmental effects of forestry on Hawke's Bay hill country. It was completed in 2005.
A paired catchment study approach was used, with one catchment in forest (Pakuratahi) and the other in pasture, farmed with sheep and beef, as a control (Tamingimingi). The catchments represented North Island hill country. Researchers compared the environmental effects of commercial forestry and pastoral farming through various stages of the forest rotation. The sequence of pre-harvest, harvest, replanting, and canopy closure covered time of major environmental change.

Harvesting effects on woody debris and bank disturbance in stream channels

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Woody debris volumes and channel bank disturbance were measured in a 100-m section of stream channel, prior to and after harvesting, in 17 streams in pine plantations in five regions of New Zealand. These sites were harvested using four different harvest methods. Volumes of pre-harvest woody debris and woody debris produced during harvest averaged 105 m3/ha, and 147 m3/ha, respectively. Apart from the stream-cleaned sites where virtually all the pre-harvest and harvest woody debris was removed, postharvest volumes (pre-harvest + harvest) averaged 289 m3/ha and increased three-fold on average over pre-harvest levels. Most of the woody debris in the stream channel was positioned above the stream—69% of pre-harvest woody debris, 64% of harvest woody debris, and 66% of total post-harvest woody debris. The remainder lay in-stream or on the floodplain. The most significant change in woody debris characteristics after harvest was size distribution. Small woody debris <10 cm in diameter (SWD) increased from 13% of woody debris volumes at pre-harvest to 38% at post-harvest. The number of pieces of large woody debris >10 cm in diameter (LWD) increased significantly, and the average length and piece size decreased significantly after harvest. This was due mainly to the removal of the larger merchantable pieces of LWD from the stream channel. Harvest method had the most impact on harvest woody debris volumes in the stream channel, overriding the influence of riparian buffers which ranged in width from 1 to 30 m at four of these sites. Stream-cleaned sites had the lowest harvest woody debris volumes, followed by sites harvested with ground-based systems (15 m3/ha and 48 m3/ha respectively). When yarder systems were used to extract timber back from the stream edge, woody debris volumes averaged 104 m3/ha, whereas hauling across the stream channel resulted in the highest average woody debris volumes of 287 m3/ha. For hauling across the stream channel only, there was a relationship between stand volume and harvest woody debris volumes. Bank collapses accounted for 68% of all pre-harvest channel bank disturbances. Bank scuffing from felling and log extraction during harvest operations was the most common channel bank disturbance after harvest (46%). Harvest method did not show a clear relationship with the degree of channel bank disturbance.

Influence of large woody debris on channel morphology in native forest and pine plantation streams in the Nelson region, New Zealand

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The influence of large woody debris (LWD) on channel morphology was assessed in five pine plantation and five native forest streams in the Nelson region of New Zealand. LWD volumes averaged 127 m3 ha–1 in pine plantation streams and 94 m3 ha–1 in native forest streams. Most of the LWD in pine and native streams had no influence on channel morphology (78 and 54%, respectively). Those LWD pieces influencing channel morphology contributed mainly to sediment storage in both pine and native streams as well as flow deflection and debris collection in the native streams. Wood aligned either perpendicular or obliquely to stream flow and positioned on or partly buried in the stream bed had the greatest influence on channel morphology. There
were twice as many pools in native streams as in pine streams, and scour processes formed the majority of pools. LWD influenced c. 50% of pool formation in both pine and native streams and increased pool variety. LWD volumes in these streams are low compared with Pacific north-west streams of the United States, similar to streams in the subalpine forest of Colorado, and higher than volumes in the more modified streams of Europe. Similarities in pool forming processes, pool type, and spatial arrangement of wood influencing channel morphology are apparent between the streams in this study and streams of the Pacific north-west and Colorado.

Stream channel erosion and change resulting from riparian forests

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Forested stream banks, compared to grassed ones, can destabilize stream channels by promoting erosion. Four reaches of Coon Creek,Wisconsin, each with long-term grassed and forested subreaches were examined. Grassed reaches were narrower and had smaller channels (bankfull cross sections) than forested reaches, suggesting that grassed channel reaches stored about 2100 to 8800 m3 more sediment per kilometre than forested reaches. Available evidence suggests that conversion of riparian forests to grass would allow storage of sediment along channels, possibly decreasing downstream sediment yields. These findings are important as many grassed riparian corridors are rapidly reverting to forest because of economic conditions and governmental policies.

Stream channels are narrower in pasture than in forest

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In the Hakarimata Range, west of Hamilton, New Zealand, second-order streams appear to be wider in native than in pasture catchments, whereas streams in pine plantations (18 years old) appear to be suffering active stream-bank erosion. A working hypothesis to explain these observations was that pasture vegetation replacing
original forest encroaches on the stream channel, causing it to become narrower. To test the hypothesis, channel widths were measured up stream and down stream of "transitions" from native forest to pasture in 20 streams of different size in marginal ranges of the Waikato Basin. Small streams (catchment area <1 km2, width in forest <2 m) were found to be half the width in pasture reaches as in forest. The degree of channel narrowing decreased as stream size increased and was minimal in large streams (catchment area >30 km2, width in forest >10 m). This narrowing of stream channels implies that native forest clearance in New Zealand has reduced stream channel habitat. A concern regarding riparian planting for stream restoration is that sediment stored in pasture stream banks could be mobilised if grasses are extinguished by shading, resulting in turbid streamwater and sedimentation of fines in the channel.

Effects of land use on the channel morphology of streams in the Moutere Gravels, Nelson, New Zealand

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Land use can affect channel morphology, especially in small catchments (up to 5-8 km^2). Previous studies have suggested that stream channels in pasture catchments are usually narrower than those in forested catchments due to the erosional resistance on grass, and its ability to trap and retain sediment. In the Hakarimata Ranges, Waikato, New Zealand, where the bedrock is predominantly sandstone, siltstone, and mudstone, streams in small catchments converted from pasture to pine plantations are in the process of widening back to forest channel morphology, releasing the sediment retained in the banks by grass sod. As the majority of new pine plantations are on reverted pastureland, the possibility on increased sedimentation instreams during this conversion process may be a problem in some areas. The Moutere Gravels in Nelson provide an area of contrasting geology, hydrology and climate to the Hakarimata Ranges for assessing the influence of land use on channel morphology. This study compared channel morphology characteristics in 15 streams in small-sizes catchments, with 5 stream catchments each in pasture, pine and native forest.
There were no significant differences in bankfull width, channel width, channel depth and cross-sectional area of streams in catchments with the tree types of land uses in the Moutere Gravels. However, width-to-depth ratios were significantly higher in the pasture streams than in the forest streams, median particle size was significantly lower in the pasture and pine plantation sites than in the native forest sites and there were more pools in the forest streams, In part attributable to the presence of woody debris.
It is suggested that low sediment yields, infrequent floods of sufficient magnitude to influence channel morphology, and the cohesive channel bank material in the Moutere Gravels may explain the lack of effects of land use on channel morphology in the small catchments. The results from the Moutere Gravels suggested that factors other than land use can sometime exert a stronger influence on channel morphology in small catchments.