The complete landscape surface of the active Mackenzie River Delta (13,135 km2) was manually partitioned into discrete lakes (3331 km2), channels (1744 km2), wetlands (1614 km2), and dry floodplain area (6446 km2) via GIS analysis of digital topographic maps recently available for the system. The census total of lakes (49,046) is almost twice as large as prior estimates. Using this new information, total lake volume in the delta during the post river flooding period is estimated as 5.4 km3. Total floodwater storage in the delta lakes and floodplain at peak water levels is estimated at 25.8 km3 and thus is equivalent to about 47% of Mackenzie River flow (55.4 km3 yr−1) during the high-discharge period of delta breakup. During this period the stored river water can be envisioned in the form of a thin layer of water (2.3 m thick on average) spread out over 11,200 km2 of lakes and flooded vegetation and exposed to 24 h d−1 solar irradiance. Consequently, this temporarily stored water has significant potential to affect the composition of river water flowing to the Beaufort Shelf as it recedes to the river channels after the flood peak.