The word 'streambed' is a transparent English compound, formed by combining two native Germanic nouns. 'Stream' originates from the Old English 'strēam', which referred broadly to a current or flow, not only of water but also of air or light. 'Bed' derives from Old English 'bedd', initially meaning a sleeping place or a plot of ground for plants, thus carrying the sense of a supportive foundation. The logic of the compound is straightforwardly descriptive: it names the supportive base or channel that contains the flowing water, much like a bed holds a sleeper or a garden bed holds plants. This literal combination of 'water current' and 'foundational layer' perfectly captures the physical geography of a river's course.

Structure: stream (flowing water) + bed (base) - stream: From Old English 'strēam' (a current of water, a flow). Germanic origin, related to Dutch 'stroom' and German 'Strom'. Functions as the core noun denoting the body of water. - bed: From Old English 'bedd' (a piece of ground prepared for plants, a resting place). Germanic origin, related to Dutch 'bed' and German 'Bett'. Functions as a noun in a compound, indicating the supportive base or channel upon which something lies.