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stage 4, might also be detected as on the Mackenzie it is often 
silt-covered. To detect a change in snow conditions would be more 
difficult, for the change in reflective capacity associated with 
seasonal changes in the snow pack are very small. Golding (1974) 
found that melting and non-melting snow can be detected on ERTS 
imagery, but only in large basins (25,000 to 30,000 Km 2 , sq), and 
involved the use of composite minimum brightness techniques. Such 
an approach is beyond the scope of this study. 
Problems in the use of ERTS imagery for the study of 
river ice break-up have been discussed by other investigators 
(Ferguson and Cork, 1972). Darkness, cloud cover, satellite pass 
dates and ice reflectance properties, are some of the difficulties 
that must be anticipated. Cloud cover, combined with the eighteen- 
day return cycle, together form the principal constraint in using 
ERTS to monitor a brief episode of fluctuating date, such as river 
break-up. Ferguson and Cork (1972) note that cloud cover and other 
factors limited monitoring success to 27 per cent of the instances 
they considered. Their report, however, deals with satellite 
imagery from sensors of inferior resolution to ERTS sensors. 
In 1973 "Quick-Look" ERTS prints were compared with 
1:250,000 scale topographic maps, using a large hand magnifying 
lens. The number of islands visible when the Mackenzie was 
completely ice-covered was compared with the number shown on the 
maps. The same comparison was made at later stages of the break 
up. The number of islands counted on imagery in April or early 
May served to establish a "baseline" for ice levels and amounts. 
Sand bars or low scrub covered islands were expected to be 
excellent indicators of rising stage or high water levels associated 
with ice dams. 
Two limitations were quickly discovered, the first with 
the-imagery and the second with the control. "Quick-Look" prints 
have poor resolution, compared with ERTS positive transparencies, 
which hindered the discrimination of islands and sand-bars from 
the background ice texture. Another major problem in interpretation 
was found when trying to distinguish between islands and open 
water areas in the later stages of break-up. Difficulties with 
control arose from using maps which were compiled as long as 17 
years ago, using aerial photography dating back to 1947 (Table 1.). 
In 1974 another attempt was made to chart the progress 
of break-up. A Bausch and Lonb Zoom Transfer Scope (Figure 3) 
was used to plot "Quick-Look" data directly onto 1:250,000 overlays. 
Optical registration of the data enabled the interpreter to check 
all questionable details directly. Although this method slowed 
the interpretation process, it was assumed that the results would 
show an improvement. Seven cloud-free "Quick-Look" frames were