406 
and for monitoring its progress. If the onset of break-up could be 
predicted from ERTS, the photographic aircraft might be deployed 
to better advantage, and ground observers at points downstream 
could make their plans more economically in an area where the 
helicopter is almost the only dependable way of field travel at 
break-up time. 
Initially we thought that the progress of break-up might 
be detailed almost exclusively from ERTS, including regional 
changes in snowmelt and the break-up on tributary streams as well 
as the stages of break-up on the Mackenzie itself. Although the 
changes in snow cover and the break-up of tributary streams, the 
Liard River in particular, are very influential in initiating 
the Mackenzie break-up, this aspect is not discussed further, as 
our knowledge of conditions in the river basin is scanty compared 
with that of the river itself. 
The break-up of the Mackenzie generally follows a series 
of identifiable stages, namely: 
1) The opening of shore leads; 
2) the development of open water areas where tributaries 
join the river, and transverse leads from shore to shore; 
3) fracture of the ice cover into large floes, with limited 
downstream movement; 
4) "flush-out", involving major ice movement, ice shove along 
the river banks, ice jams and the progressive fracture of 
ice floes into smaller pieces, until brash ice becomes 
the main ice type; and 
5) brash ice has been flushed out, and the river is almost 
clear of floating ice. A considerable amount of ice is 
still stranded on channel banks and islands, and may 
interfere with the loading or unloading of barges. 
Although ERTS "Quick-Look" imagery probably would not 
pick up the first stage, shore-lead development, it seemed probable 
that flooding of islands by the rising river would be visible. 
Low-lying islands would be of particular importance since these 
would be the first to be flooded. As shore-leads, transverse 
leads and extended melting occurred around the shorelines of the 
islands, this expanding store of visible evidence for predicting 
the initial movement of the ice cover might be detected from an 
examination of ERTS imagery. Overturned ice debris, typical of