Lahars at Semeru volcano, Indonesia, are an ongoing phenomenon that rapidly transport large amount of clastic materials, threatening populations and infrastructures on and at the foot of the volcano. Focusing on lahars' deposits, this contribution has three main aims: (1) Understand the terrasses and valley bottom deposits architecture and their eventual correspondence with GPR (Ground Penetrating Radar) electromagnetic signals; (2) Calculate volumes of removable materials inside the valley; (3) explain the deposits irregular distribution in the valley. In order to reach these goals, we worked on a 4.2 km stripe of the Curah Lengkong valley, 8 km S-SE from the summit in an area favorable to lahar deposits. We used a geomorphological approach, completed with an extensive GPR campaign over the terrasses and deposits at the bottom of the valley. We also analyzed a series of aerial photographs that we converted into GIS in order calculate lahars' terrasses volumes, and in order to understand their spatial distribution. Results highlight that the terrasses' architecture is typically divided into horizontal units, with “progradation like” lateral variations, whereas deposits at the bottom of the valley are not presenting any clear architecture. The calculated volume of lahar deposits is 0.51 million cubic meter for the 4,2 km channel section we studied. This overall volume is divided between 210,200 m3 for the terrasses and 301,000 m3 for the bottom of the valley. These deposits are unevenly spread through the valley since they are mostly concentrated in the upstream half of the valley, with 25% of the material located within the first 500 meters. We could identify to concentration areas that are due to (1) a natural topographic jump and (2) the presence of a SABO dam that blocks the sediments. Therefore, this study emphasizes the role of local topography on deposits, and the importance of the “invisible” materials located at the bottom of the valley.