The 2011 Tohoku-oki tsunami inundated the Idagawa estuarine lowland in Odaka District, Minamisoma City, to a distance of up to 3.2 km inland from the shoreline. In this study, the formation processes of the tsunami deposits is reconstructed based on the pathways of the tsunami, facies of the tsunami deposits. The tsunami deposits are divided into three main units (Units 1-3, in ascending order), and Unit 1 is subdivided into Subunits 1A-1C. Subunit 1A consists of fine sand and was deposited by the encroached flow through the drainage channel in the earliest stage of the tsunami. Subunit 1B comprises an assortment of gravels and was formed from the overflow of a sea dike and river bank. Subunit 1C consists of medium to fine sand and was formed from the flooded flow. Most of these subunits formed from the run-up flow of the first tsunami wave with a remarkably high water level. However, clear erosional contacts are occasionally recognized in the subunits, which were formed from minor run-up flows after the secondary wave. Unit 2 consists of poorly sorted muddy fine sand and was formed from the return flow due to a drop in water level in the latter half of the tsunami. Unit 3 consists of massive mud settled down from the ponding water after the tsunami.

The Tohoku-oki tsunami inundated the Atsuma-gawa River on the Pacific coast of Hokkaido, Japan, to. km inland from shore line and left tsunami deposits on the sand bars. In this study, we reconstruct the formation of the river tsunami deposits using sedimentology, and observations of the development of the tsunami waves. We divide the tsunami deposits into three main units (Unit to , where Unit is the lowermost deposit). Unit comprises a mixture of shallow marine sand and fluvial deposit, involving fallen plants at the base indicating the current direction towards the sea. We interpret Unit as a return flow deposit; extensive return flow occurred after the initial inundation, following a large drop in the level of the tide and rapid erosion of river mouth bar blocking the river channel. Unit consists of shallow marine sand, which rapidly thins and fines inland, and cross laminae indicate the sand was transported by landward flow. We interpret it mainly as a run-up deposit, which formed from tsunami waves that encroached inland, losing energy, wave amplitude, and velocity. Unit comprises alternating layers of shallow marine sand and silt (mud drapes), and represents small variations of the grain size between seaward and landward. Current directions estimated by cross laminae indicate mainly landward, and partially seaward. We interpret Unit formed from repeated long-period runup and return flow cycles in the latter stages of the tsunami, and mud drapes were deposited from fluid mud. Return flow concentrated in topographic lows and the channel. By documenting the formation processes of tsunami deposits correlate with recorded tsunami waveforms, we aim to improve our understanding of how tsunamis interact with onshore river systems, in Japan and elsewhere.