Discussions and Reply
Reply to Mussetter::
There are limited data available on mud accumulations in pore spaces of streams
in the study area of NW Branch. However, surveys of fine-grained sediments in pores
have been completed along a 24-km reach of the South River in the Shenandoah Valley
of Virginia (Pizzuto et al., 2006). Along South River, an ongoing study of mercury
contamination has necessitated monthly sampling of fine-grained sediments in a variety
of environments, including within the porous gravel stream bed. Sediments are sampled
at the downstream ends of pools in riffle-pool sequences, where deposition is likely.
Samples are obtained from 12 locations along the study reach. At each site, samples are
taken from each of three 0.5 m quadrants equally spaced across the width of the stream.
Pore water and sediment are pumped from the subsurface within the quadrat using a
hand-held Beckson bilge pump; the resulting slurry is collected into a bucket. Coarse
sediment is allowed to settle, so only the silt and clay fraction is sampled. The volume
of the slurry is recorded.
Table 10.1 presents volumes of silt and clay sampled from the bed at 12
locations in the study area in March and April 2006. These volumetric data were
converted to units of mass per unit bed area (M) using the following equation:
M  LV ss  / As
where L is the “wet” volume of sediment sampled (the samples listed in Table 1 were not
dried, and therefore the volume listed includes both water and sediment), Vss is the ratio
of solid volume to total volume of the sample,  is the density of the sediment, and As is
the area of the bed sampled. Values of these parameters used in the conversion are listed
in Table 2.
Table 1. Volume and mass per unit bed area of silt and clay stored in the bed of South
River at 12 locations sampled in March and April 2006 (Pizzuto et al., 2006).
Table 2. Parameters used to convert volumes in Table 1 to mass per unit bed area.
Parameter
Value
Volume sampled (gallons)
15
2
Bed area sampled (ft )
36
Slurry solid volume ratio
0.5
Unit weight of sediment (kg/m3)
1800
The average mass of silt and clay stored in the bed for all sites and both sampling
dates is 0.05 kg/m2. These data suggest that considerable amounts of fine-grained
sediment are indeed stored in gravel pores in mid-Atlantic streams of the East Coast,
U.S.A. These data, of course, are hardly comprehensive, and although subsequent
monthly sampling through December, 2006 confirms that these values are not unusual,
much additional study is needed. Furthermore, these rather generalized observations are
not sufficient to confirm the model used in the present simulations. Rather, they simply
suggest that storage of silt and clay in pores of a gravel bed may be significant in the
region. Detailed confirmation of the model used in this paper must await additional
detailed study.
The author is grateful for this discussion. Apparently there are important regional
differences in how fine-grained sediments interact with gravel streambeds that should be
investigated further.
Reply to Reid:
The author appreciates the opportunity to clarify the calculation procedure used in
the model. The grain size distribution of the bed is updated at each time step, before any
attempt is made to compute the silt-clay content of the gravel streambed. Thus, the
model computations do reflect and hopefully account for changes in the texture of the
streambed through time. This does not mean, of course, that the computations are
actually correct or even realistic in detail, which could only be established through a
detailed observational study in the field or laboratory.
As the discussion indicates, no attempt has been made to explicitly account for
mesoforms such as bars or pools and riffles. The detailed morphology of the channel
wetted perimeter is certainly very likely to influence the extent of storage of silt and clay
in gravel-bed rivers. Indeed, field studies by our research group on a gravel bed river in
Virginia, U.S.A., implicate large woody debris accumulations in pools as the primary
control on the extent of fine-grained sediment storage at this field site (Skalak and
Pizzuto, 2006; Pizzuto et al, 2006). However, knowledge of “mesoscale” storage
processes does not appear adequate at present to allow creation of a numerical model of
these processes.
Reply to Vide:
The balance between sediment transport capacity, supply, and adjustment of the
stream attributed to Lane that provides the basis for this comment applies to alluvial
streams that are free to adjust their morphology. However, a variety of field observations
suggest that streams of the study area are strongly influenced by bedrock exposures in the
stream bed (Skalak, 2004), and by riparian vegetation that armors river banks
(Allmendinger et al., 2005, Pizzuto and Meckelnburg, 1989). These streams are likely
not entirely free to adjust their morphology, and should not be considered as traditional
“alluvial” channels, but rather as channels that exhibit features of both bedrock and
alluvial streams. For example, while urbanization has resulted in extensive incision in
some areas (Trimble, 1997, Booth, 1990), many streams in mid-Atlantic urbanized
watersheds have not incised deeply, and in many cases, incision cannot be demonstrated
to have occurred at all (Hammer, 1972, Pizzuto et al., 2000).
The model described in the paper includes this influence by representing bedrock
explicitly as a dynamic component in the active layer of the bed. The author appreciates
the opportunity to explain this aspect of the model in greater detail.
Reply to Rennie:
This discussion raises very important issues related to our ability to forecast the
influence of climate and land use changes on rivers. The discussion notes that historical,
stratigraphic studies lack detail, while modeling studies are generally poorly constrained.
Our study was motivated by the need to provide detailed geomorphic information
that could be used to model the influence of climate and land-use changes on fish
populations in headwater streams of our study area. This goal required us to predict the
grain size distribution of the bed, the fine-grained sediment content of the pores, the
turbidity caused by suspended sediment, and flow velocities caused by a variety of
discharges. While many of the processes included in the model could be verified by
experimental or field observations, the boundary conditions required by the model that
relate land use and climate changes to sediment supply and discharge are virtually
untestable in significant detail, either by stratigraphic study or any other approach. The
best one could really hope for is an adaptive modeling approach where model predictions
are periodically adjusted after comparing model predictions to long term monitoring of
changes observed in the field as they occur over decades.
The discussion implicitly questions the value of modeling long term geomorphic
processes influenced by changes in climate and land use. The author answers such
questions by explicitly noting the difference between scientific goals and the necessity of
making management decisions. The goal of science is to achieve an understanding of
nature that is supported by observation. This goal almost by definition cannot be
achieved by modeling alone, particularly when models are applied to future conditions.
In the author’s view, the present study (and others like it) represents an extension of
traditional scientific goals to provide “best guess” working hypotheses to managers for
decision making purposes. Scientists always have the obligation, of course, of including
realistic evaluation of model uncertainty when providing results such as these.
In the discussion, it is suggested that models might be best evaluated by
reproducing historical events. As noted, past changes to fluvial systems are accessible, at
least in outline form, though stratigraphic studies. This approach indeed provides some
of our best current opportunities to evaluate models of long term channel changes.
Physical modeling studies can also prove useful in some instances. Hopefully, though,
some selected areas will be chosen for long term observational studies that can provide
the field data needed to evaluate future predictive models.
References:
Allmendinger, N. E., Pizzuto, J. E., Potter, N., Johnson, T. E., & Hession, W. C. 2005.
The influence of riparian vegetation on stream width, eastern Pennsylvania, USA.
Geological Society of America Bulletin. 117, 229-243.
Booth, D.B. 1990, Stream-channel incision following drainage-basin urbanization.
Water Resources Bulletin, 26: 407-417.
Hammer, T.R., 1972. Stream channel enlargement due to urbanization: Water Resources
Research, 8, 1530-1540.
Pizzuto, J. E. and Meckelnburg, T. S. 1989. Evaluation of a linear bank erosion
equation. Water Resources Research, 25:1005-1013.
Pizzuto, J.E., Hession, W.C., and McBride, M., 2000, Comparing gravel-bed rivers in
paired urban and rural catchments of southeastern Pennsylvania, Geology, 28, 79-82
Pizzuto, J.E., Skalak, K., O’Neal, M., Narinesingh, P., Rhoades, E., and Hess, J. 2006.
Geomorphology of the South River between Waynesboro and Port Republic Virginia:
Geomorphic characterization and annual budget of silt and clay. Unpublished technical
report, 14 chapters.
Skalak, K. J. 2004. The effects of dam removal on downstream channel characteristics:
assessing the potential consequences of dam removal. M. S. Thesis, University of
Delaware, Newark, Delaware. 133 p.
Skalak, K.J, and Pizzuto, J.E., 2006, Fine-grained channel margin (FGCM) deposits
conditioned by Large Woody Debris (LWD) in a gravel-bed river. EOS Transactions of
the American Geophysical Union, 87(52) Fall Meeting Supplement, Abstract H41H-05.
Trimble, S.W., 1997. Contribution of stream channel erosion to sediment yield from
an urbanizing watershed: Science. 278, 1442-1444.