Commonwealth Forestry Review

42 COMMONWEALTH FORESTRY REVIEW
contrasted with the mineral sediment from the streams after they had flowed through
the logged and roaded area.
The sediment graphs from the sampling stations located below logging activity
shown in Fig. 3 show a peak early in the rise of the stream. This pattern was also
characteristic of almost all the sampling in subsequent years and was probably due to
the sediment source areas being fairly close to the sampling stations. The initial
flush of surface runoff from bare areas in the lower portions of the catchment caused
high sediment concentrations in the early stages of the stream rise. These high
concentrations were diluted as the increased flows from the undisturbed upper catchment
areas made their impact on the total flow. Sediment concentrations then increased
again due' to stream bank and bed erosion from the high flows'.
Samples were taken from one of the reservoirs in the city of Cairns nine hours after
the main storm period which produced the sediment pattern shown in Fig. 3. These
samples revealed a concentration of 430 p.p.m. The reservoir became fully charged
with turbid water and took about a week to clear.
Data from the permanent sampling stations provided information on the total
sediment levels from the disturbed area of the catchment. However, 'it was necessary
to pinpoint the sediment source areas and evaluate the contributions of the various
sources. To achieve this, samples were collected from sections of the stream which
were crossed by roads or were influenced by snig tracks, log ramps, etc. In some cases
sediment source areas were located by following turbid water upstream during rain
periods, until the source of the sediment was found.
It became apparent that most of the sediment was derived from poorly located
undrained roads and snig tracks and from earth and log fill crossings such as that
shown in Fig. 4. One striking point which emerged was the fact that a major portion
of the sediment was derived from a few small sediment source areas. An example of
the effect of one of these areas is shown in Table 1. Even though no noticeable rise
occurred in stream height, considerable sedimentation resulted.
TABLE I
Sediment concentration from a log ramp adjacent to the stream after rain influence of 26
points in 20 minutes between 1320 and 1340 hours on 25.1.66.
Upstream of ramp
Location Time (hours) Sediment concentration (p.p.m.)
Immediately downstream of ramp
50 metres downstream of ramp
400 metres downstream of ramp
400 metres downstream of ramp
13.45
13.45
13.50
14.00
15.45
48
2,602
Table 1 shows also how the sediment clears rapidly once rain ceases. However,
during the wet season light showers such as the one which caused the sediment levels
shown in Table 1 frequently occur for days or even weeks at a time. Consequently
this can result in an almost continuous source of sediment-laden water leaving the
catchment.
In general, sedimentation occurred following almost every rain influence and it
became severe and affected the city water supply after rains ofmoderately low intensity.
The area covered by snig tracks during normal rainforest logging is considerable
as can be seen from Table 2. '
203
186
21