D. Murray Hicks

Organic carbon fluxes to the ocean from high-standing islands

The transfer of terrestrially derived (yet poorly quantified) organic carbon to the oceans is an important yet unagreed upon proportion of the worlds carbon budget. The few data that exist suggest that high-standing oceanic islands in the southwest Pacific may make important contributions to the overall terrestrial flux of particulate organic carbon (POC) to the oceans. We have determined the POC flux from several streams in New Zealand. With those data and previous measurements from Taiwan and Papua New Guinea, we estimate the POC flux from high-standing islands in the southwest Pacific to be 48 × 1012 g C yr−1, a value slightly lower than previous estimates. These islands, which make up only ∼3% of Earths landmass, contribute 17%– 35% of the estimated POC entering the worlds oceans annually and thus may be important sources of terrestrial carbon to the ocean. Anthropogenic activities, especially deforestation and forest harvesting, have probably exacerbated the natural flux. Few to no data exist for many of these islands and thus a more detailed assessment awaits further measurements.

Riparian Vegetation as a Primary Control on Channel Characteristics in Multi-Thread Rivers

Vegetation has been recognized as a primary control on river planform, particularly as a determinant of whether a river will adopt a braided or single-thread pattern (e.g. Millar [2000]). Studies have shown that overall behavior of the system correlates with vegetation type or density, shifting between a single-thread channel and a multi-thread system as vegetation changes [Mackin, 1956; Brice, 1964; Nevins, 1969; Goodwin, 1996; Ward and Tockner, 2000]. Murray and Paola [1994] concluded that braiding is the main mode of instability for unconstrained flow over a noncohesive bed. In other words, in the absence of cohesion to stabilize the banks and/or discourage formation of new channels, the flow tends to create new channels until a braided system develops. There-

Event Suspended Sediment Characteristics and the Generation of Hyperpycnal Plumes at River Mouths: East Coast Continental Margin, North Island, New Zealand

Steepland rivers draining small, coastal watersheds often have very high suspended sediment loads, but the event characteristics of suspended sediment concentration and yield in this class of river is not well documented. Continuous monitoring at four sites in the Waipaoa River basin, New Zealand, demonstrates that during individual and composite events, suspended sediment concentration versus water discharge relations typically show clockwise hysteresis and that event maximum concentration is poorly related to event peak discharge. The signature of different erosion processes is also imprinted on the event yield magnitude frequency distributions. Gully‐dominated tributary basins produce relatively high yields at all frequencies, reflecting greater sediment availability, whereas in tributary basins, where shallow landsliding is the dominant erosion process, there is a steep increase in yields in relation to return period. We estimate that flood discharges from the Waipaoa River approach or exceed the critical suspended sediment concentration (∼40,000 mg L−1) for hyperpycnal plume generation (because of negative buoyancy) at the river mouth once every ∼40 yr, but in the neighboring Waiapu and Uawa Rivers, the threshold concentration may be exceeded once a year and two to three times a year, respectively.

Downstream fining in a rapidly aggrading gravel bed river

Downstream changes in particle size that occur in the Waipaoa River, a 104-km-long gravel bed river in which rapid aggradation in the historic (post-1800) period was triggered by the conversion of native forest to pasture, are summarized in this paper. The textural data presented are unique for a field situation, not only because of the spatial resolution and extent of the sampling program but also because they provide information about the pattern of fining at different points in time. They are supported by equally comprehensive topographic survey data from which local rates of aggradation can be derived. Despite variability induced by lateral sediment inputs, there is an essentially continuous pattern of fining along the entire length of the river. Fining occurs in both the fine and coarse size fractions of the bed material. The highest rates of fining occur in the larger percentiles of the subsurface bed material and in the surface bed material. Downstream fining in the Waipaoa River appears to be a response to changes in flow hydraulics that are regulated by the concave configuration of the long profile. The fining gradient developed rapidly (in <45 years). It does not appear to be influenced by the rate of aggradation (nor the overall rate of sediment supply to the channel system), because, in the short term, aggradation has a negligible impact on the inherited form of the long profile.

Contribution of floodplain sequestration to the sediment budget of the Waipaoa River, New Zealand

Abstract Rapid vertical accretion on the Waipaoa River floodplain is conditioned by the river’s high suspended sediment load (30 000–40 000 mg l−1 at flood stage). Cumulative sediment accumulation curves derived from three cores suggest an average (post-1850) rate of vertical accretion of c. 60 mm a−1, though a 15 year lacuna in flood activity has depressed the post-1948 rate to c. 40 mm a−1. Rates of aggradation during floods are several orders of magnitude larger than the time-averaged rate. Within a 44 km long reach, cross-section surveys indicate that 0.2–0.8 m of sediment was deposited between 1979 and 1990. Over this period floodplain storage accounted for 5% of the total suspended sediment load, and 16% of the suspended sediment load transported during events that exceeded bankfull stage. The Waipaoa River floodplain may be representative of floodplains bordering rivers with high suspended sediment loads, produced by rapid, episodic vertical accretion, on which overbank deposition occurs across the entire floodplain, and is complemented by channel aggradation. Such rivers are able to construct high banks. Thus channel capacities are greater and the incidence of overbank flows is less than in rivers where overbank deposition is slow relative to the rate of floodplain destruction by lateral migration. The difference between our time-averaged estimate for sequestration on the Waipaoa River floodplain and comparable estimates for actively meandering rivers, and meandering rivers with low sediment loads, reinforces the notion that there is a link between the sediment transport regime of a river and its sedimentary record. To elucidate this link it is necessary to view vertical accretion in the context of the flood events that generated it, rather than in the context of a time-averaged sediment budget.

21 Contemporary morphological change in braided gravel-bed rivers: new developments from field and laboratory studies, with particular reference to the influence of riparian vegetation

Abstract Contemporary (event to decadal-scale) morphological changes in two large braided rivers in Canterbury, New Zealand, are described, along with laboratory studies that support the field observations. In the process, some new developments in field and laboratory methods for investigating morphological change in braided rivers are presented, and Paolas (2001) hypothesis that braiding tendency should be influenced by a rivers ability to turn over its bed within the characteristic time for riparian vegetation to establish and grow to a mature, scour-resistant state is examined. The lower Waitaki River has been regulated for hydropower since 1935, and since then vegetation has encroached over the riverbed and braiding intensity for a given discharge has reduced. Measurements of vegetation removal by floods indicate that floods are not able to turn over the bed fast enough to contain vegetation encroachment, and the present braiding state is held by virtue of a regular spraying programme. In contrast, on the unregulated and sparsely vegetated Waimakariri River, remotely sensed high-resolution topographic surveys using LiDAR and digital photogrammetry have shown that even sub-annual floods turn over large proportions of the braidplain. The laboratory studies show that a braided river will evolve into a single-thread channel when its bed is invaded by vegetation and floods are too infrequent to contain the vegetation growth. Collectively, the field and laboratory evidence confirms that Paolas (2001) dimensionless time-scale parameter is a reasonable first-order predictor of whether floods or vegetation will achieve ascendancy, driving a river towards either braided or single-thread end-points, respectively.

The efficiency of depth-integrating samplers in sampling the suspended sand load in gravel bed rivers

Abstract The depth-integrating sampler approach for measuring the velocity-weighted mean concentration of suspended sand in rivers was compared with the point-sampler approach whereby vertical profiles of velocity and sediment concentration are measured then integrated. The aim was to determine, for sand suspension in gravel bed rivers, the uncertainty induced when the depth-integrating sampler traverses the near-bed zone of high sand concentration too quickly to average-out the dominant fluctuations in sand concentration associated with turbulence. Depth-integrated and point samples, velocity profiles, and turbulence measurements were collected from three gravel bed rivers at various flood stages. For two rivers, the sand and silt-clay fractions of the suspended load were determined, while at the third river four separate sand size fractions and the silt-clay fraction were analysed. The results showed that the agreement at-a-vertical between the two approaches improved exponentially as a function of the shear velocity to fall speed ratio, u ∗ /w , ranging from up to ±70% for u ∗ /w down to about ±5% for u ∗ /w > 30 . The exponential trend is consistent with diffusion models for suspended sediment vertical distributions which predict that the mixing increases as a function of the Rouse number w/(B ku ∗ ). Thus when the u ∗ /w ratio is close to 1 the suspended sand load is concentrated near the bed and a depth-integrating sampler will only sample it for a small fraction of its total traverse time; conversely, with a large u ∗ /w the sand will be mixed over the flow depth and it will be sampled for the same time by a depth-integrating sampler as by a point sampler. The agreement between the two approaches appeared to saturate at about ±5% at high values of u ∗ /w in response to temporal variations in sediment concentration and flow velocity longer than the sampling time of either sampler, i.e. of the order of 1 min or more.

Towards understanding river sediment dynamics as a basis for improved catchment, channel, and coastal management: the case of the Motueka catchment, Nelson, New Zealand

ABSTRACT This paper brings together work in the Motueka catchment that has focused on both suspended sediment data and bedload transfers to provide a more holistic understanding of sediment dynamics in the catchment to inform effective river management. The annual suspended sediment load averages 349,000 t and shows considerable temporal variability (49,000 t to 1.7 Mt). Event yields may increase by an order of magnitude in response to single high magnitude storm events. Much of the sediment is generated from high rainfall areas of the catchment under indigenous forest and grassland. Short-term studies show pasture areas have a higher specific sediment yield than production forest, but that forest harvesting leads to a short-term increase in yield. Bedload transfers assessed via morphological budgeting from digital elevation model (DEM) differencing in selected reaches of the upper Motueka reveal similarly highly variable transfers at an annual scale, reflecting the magnitude and frequency of competent flow events. Longer term mean bed-level (MBL) changes demonstrate a high degree of spatial variability in the upper Motueka. Overall, DEMs of difference and longer term MBL changes both reveal a net channel degradation and export of bedload in the mainstem of the upper Motueka. Suspended sediment data also suggest an overall reduction in sediment yield from the catchment, suggesting a catchment-wide limitation of sediment supply, or a period of lower flows reducing sediment mobilization. This understanding has informed on issues such as the role of river channel management and catchment land use on in-stream ecosystems, coastal erosion, and shelf water quality and fisheries. Future river management, if it is to be effective, needs to recognize the history of this system, its likely longer term trajectory, and its linkages with the coast.