There are very few (if any) river beds that are characterized by ~1-5 mm gravel particles on Earth. That is not to say that these grain sizes do not exist, but that they rarely form the dominant grain size of river beds. An excellent place to explore the grain size gap in more detail is at the gravel-sand transition. At the gravel-sand transition, river bed sediments rapid fine from gravel (~10 mm) to sand (~1 mm), often over just a few channel widths. River bed sediments usually fine gradually downstream. The gravel-sand transition is the only exception. In this seminar, I will demonstrate the wider effects that this abrupt grain size transition has on alluvial river dynamics in terms of channel migration, floodplain recycling and why it is important to recognise it in terms of detrital sediment sampling downstream of mountain ranges. I will then consider the different mechanisms through which gravel-sand transitions and the grain size gap emerge using observations from new physical flume experiments. These new observations suggest that stable gravel beds of ~1-5 mm cannot exist where sand is being transported as suspended bed material load.