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Tunnel valleys have been widely reported on the bed of former icesheets and are considered an important expression of subglacial meltwaterdrainage. Although known to have been cut by erosive meltwater flow, thewater source and development of channels has been widely debated; rangingbetween outburst flood events through to gradually occurring channelpropagation. We have mapped and analysed the spatial pattern and morphometryof tunnel valleys and associated glacial landforms along the southern sectorof the former Laurentide Ice Sheet from high-resolution digital elevationmodels. Around 2000 tunnel valleys have been mapped, revealing an organisedpattern of sub-parallel, semi-regularly spaced valleys that form indistinctive clusters. The tunnel valleys are typically  < 20 km long,and 0.5–3 km wide, although their width varies considerably down-valley.They preferentially terminate at moraines, which suggests that formation istime dependent; while we also observe some tunnel valleys that have grownheadwards out of hill-hole pairs. Analysis of cross-cutting relationshipsbetween tunnel valleys, moraines and outwash fans permits reconstruction ofchannel development in relation to the retreating ice margin. Thispalaeo-drainage reconstruction demonstrates incremental growth of mostvalleys, with some used repeatedly or for long periods, during deglaciation,while others were abandoned shortly after their formation. Our data andinterpretation support gradual (rather than a single-event) formation ofmost tunnel valleys with secondary contributions from flood drainage ofsubglacial and or supraglacially stored water down individual tunnelvalleys. The distribution and morphology of tunnel valleys is shown to besensitive to regional factors such as basal thermal regime, ice and bedtopography, timing and climate

Morphological properties of tunnel valleys of the southern sector of the Laurentide Ice Sheet and implications for their formation