The chemistry, mineralogy, and rates of transport of sediments in the ...

13
changes in albedo, and the general availability of easily erodible
terrain . Rivers with high values of,b are interpreted to exert a great
erosional pressure on their watersheds, and can transport large quantities
of sediment . Rivers with loww values of b appear to exert less energy
in eroding their watersheds, and appear to have a lower capability to
transport sediment . This interpretation of b is partly supported by a
statistically significant (r .- 0.96, 1-a >99 .9) logarithmic relationship
between b (Table 2a) and mean annual sediment erosion rates per unit
watershed area MW , see Table 3) for the sampled rivers .
Watersheds with high values of the intercept (a) in Fig . 1 are in
regions of low relief with relatively thick deposits of alluvial, .eolian,
glacial, and/or lacustrine sediments (Rutter et al ., 1973 ; Zoltai and
Pettapiece, 1973 ; Tarnocal, 1973 ; Zoltai and Tarnocai, 1974) . These small
to moderate sized watersheds contain relatively low energy streams that
carry relatively low SSi as they slowly cut through easily erodible terrain .
Per unit increase in Qj, however, these high a watershed streams carry a
larger increment of SS, (i .e., the arithmetic slopes of selected tangents
to the curvilinear relationships between SSi and Qi, as-distinct from
the logarithmic relationships of SSi and,Qj in Fig . 1 and Table 2a), compared
to low a watersheds . This implies . that, for a small increase in QV
a unit of terrain of high a watersheds yields more sediment to the stream
than does a unit of terrain of low a watersheds, but few high a water-,
sheds in Table 2a have the sustained energy of high QQ (i .e ., b) to
transport this easily eroded sediment .. This further implies that the
stream beds of high a and low b watersheds should contain a greater range
of particle sizes, particularly a greater proportion of sand, silt and clay .
Watersheds with low values of a and high values of b would presumably scour
their channel beds clean of clay, silt,'and fine sands, leaving only gravel
and boulders (such as in the case of the main channels of the Mackenzie,
Liard, Peel, South Nahanni, and Arctic . Red Rivers) . Watersheds with high
values of a (i .e . easily erodible watersheds) and low values of b (less
capacity to transport sediments) probably supply sediments from the land
drainage area in excess of the ability of the stream to carry the sediments
away, resulting in periodic accumulations of sand, silt, and clay amongst
the boulders and gravel of the str?am bed (such as in the case of the Harris,
Martin, and Willowlake Rivers) .
If the above interpretations of the slopes and intercepts of logarithmic
relationships between,SS- and Q, are tentatively accepted, some interesting
speculations regarding the biological utilization of the stream beds can be
made . If zoobenthic species diversity is related to substrate diversity,
then streams and rivers with low slope (b) values and high intercept (a)
values should have a greater diversity of zoobenthos than rivers having
high values of band low values of a .' This cannot be directly,tested with
our present zoobenthic data (Rosenberg and Snow, 1975 ; Brunskill et al .,
1973) because of the lack of suitable quantitative zoobenthos and sediment
sampling methods for large rivers . There is some partial support in Brunskill
et al . (1973, vol . 2, p. 307-309) and Campbell
et aZ .,(1975, Table la in
.Section 5) for the hypothesis that rivers of low slope (b) will have a