Two pool-to-pool spacing periods on large sand-bed rivers: Mega-pools on the Madeira and Mississippi

Abstract The Madeira River thalweg includes two pool-to-pool spacing periods. First, standard pool-crossing sequences form 6 to 8 m deep pools. These pool-crossing sequences recur approximately every six channel widths (9.7 km or 6.8 W), on average. However, the Madeira study reach (between Manicore and Porto Velho in Northern Brazil) is relatively straight, making these pools more frequent than meanders. Second, much deeper pools (13 to 30 m deep) also form at relatively regular, but much longer spacing (50–60 km, about 40 channel widths apart). The deeper “mega-pools” occur where the river reaches the edge of the Holocene alluvium, encountering resistant units (e.g., granite, mud clasts, and conglomerates) in the confining Pleistocene units. These two pool scales also occur on the Lower Mississippi River (downstream of Tarbert Landing). This study identified pool formation processes responsible for these two pool-depth and –spacing scales on two large sand-bed rivers. The analyses computed pool statistics and correlated the deepest pools with planform controls. Two distinct pool formation hypotheses explain the two pool types. The standard pools are “rhythmic” features, developed by macro-turbulent feedbacks, while the mega-pools are “forced” features imposed by the regional geology. The former are fundamentally periodic; the latter are not. However, periodogram analysis indicates that mega-pools on the Madeira form a second, relatively regular pool period. The regional geology, including the confining Pleistocene bluffs and, potentially, orthogonal neotectonic lineaments, imposed periodic regularity to these mega-pools on the Madeira River that is not observed on the Mississippi River, a comparably-sized sand-bed river with mega-pools.