Abstract: Objectives: Ice-velocity for Nioghalvfjerdsfjorden Glacier (79NG) and Zachariæ Isstrøm (ZI), the two main outlets of northeast Greenland, were produced to clarify why neighbouring glaciers exhibit distinct seasonal speed-up patterns under similar climatic forcing. Methods: Six-day, 80-m ice velocity for 2017–2021 were derived from 598 Sentinel-1 A/B images with offset tracking, corrected by stable bedrock targets and validated against ITS_LIVE and PROMICE products. Concurrent daily series of sea-ice concentration/thickness, sea-surface temperature, 2 m air temperature and catchment runoff were analysed through Pearson and partial correlations after 30-day smoothing. Results: 79NG peaks at 1.4 km a^-1 at the grounding line and decelerates along its 80 km floating tongue; ZI accelerates continuously to 3.2 km a^-1 at the terminus. Summer velocity surges of 79NG consistently precede runoff maxima by 2–4 weeks and coincide with rapid June break-up of the seasonal ice mélange. In 2018 and 2020, such events advanced the speed-up to late May, whereas in 2017 and 2019 thick land-fast ice delayed it to mid-June. In contrast, ZI velocity correlates strongly with runoff while sea-ice remains quasi-permanent. An anomalous anticyclone in June 2019 melted the mélange within five days and drove the largest velocity of 79NG. Conclusions: Mechanical buttressing by the sea ice governs 79NG short-term dynamics, whereas ZI flow is controlled by runoff-enhanced basal sliding. Continued warming and recurrent mélange break-up will therefore prolong 79NG seasonal acceleration and increase its sea-level contribution.