Rivers in the humid tropics are those with the largest discharges and sediment loads of the world. Their evergreen and ever wet catchments are hotspots of biodiversity and their fertile deltas are acres of plenty for the production of rice, palm oil and rubber. At present, tropical rivers, their catchments, and deltas face growing pressure from rapid economic development and climate change, which may permanently deteriorate their ecosystem services. Yet,despite their importance and advancing degradation, relatively little is known about their physiography. This thesis reduces this gap by contributing to our fundamental understanding of tropical rivers.This thesis in particular addresses fundamental questions regarding the hydro- and morphodynamics of large sand-bedded rivers and their tidally influenced deltas: How can river and tidal discharge be effectively measured? How do the cross-section geometry and bed material change along the fluvial-tidal transition? How do these trends differ between the distributaries? How does the tide propagate up-river river? How can sediment transport be efficiently measured with acoustic instruments? How are the discharge and the sediment divided at river bifurcations? To address these questions, the author undertook a year-long journey to West Kalimantan, during which he surveyed and monitored the Kapuas River. The Kapuas River is nearly pristine and thus gives a rare insight into the hydro- and morphodynamics of a river that has not yet been restricted by either dams, dykes or groins. Findings from the survey of the Kapuas River are generalized with idealized models.