The glacier flow (let us designate it as Q) through any transverse cross-section adds to the mass of the glacier below the cross-section. Whatever the transport of substance through the cross-section in a given interval-of time (let us say, in a year), the same amount of substance is received by the downstream part of the glacier. This accession, by increasing the volume of the downstream part, is reflected in the height of the glacier surface. The volume of the accession (Q) may be defined as the product of the area of the downstream part by the mean rise of its surface. At a time when no melting of the glacier is in progress, the rise in level, due to the influx from above, may be taken as equal to the difference between the mean surface heights of the said part of the glacier at the beginning and at the end of the season.
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