Tidal Waves in rivers: I assume they were talking about something like the Severn Bore which is popularly known as a tidal wave. (not sure about the "down" part, unless it was on its way back to the sea, having hit a barrier of some kind)

According to the Environment Agency (the government department with responsibility for water and waterways):
"A tidal wave, or bore, occurs in the lower reaches of a few rivers during high tides. For a bore to form, a considerable rise in tide is needed in a converging channel with a rising bed, forming a funnel shape. These conditions occur in the lower reaches of the River Severn, forming the Severn Bore.The Severn Estuary experiences the second highest tide anywhere in the world, with a range which can be in excess of 14.5 metres. Under the most favourable conditions, the Severn Bore may reach two metres in height. Opposing winds or high freshwater levels can considerably reduce the height and delay the time of arrival, whereas a following wind can increase the height and advance the time.
The average speed of the bore is approximately 16 kilometres per hour." Here is a picture: http://members.tripod.com/~BoreRidersClub/NEW100.jpg

An excellent site explaining the phoenomenon:
says: "People often erroneously assume that a bore is caused by the tide itself. A bore occurs as part of the tide - it is in fact (as will be outlined below) the wave(s) at the head of the incoming tide when it has entered an estuary - and hence cannot be caused by the tide. Another fatal error is to describe the bore as a 'tidal wave'. 'Tidal wave' refers to any surge of the sea occurring through factors independent and mutually exclusive of the tide (especially meteorological disturbances or earthquake). Hence a 'tidal wave' (tsunami or seiches) is not a tide and therefore cannot be associated with a bore." http://members.tripod.com/~BoreRidersClub/Theory.html