When a river mouth has a flat, converging shape and when the tidal range exceeds 6 to 9 m, the river may experience a tidal bore (Video). A tidal bore is basically a series of waves propagating upstream as the tidal flow turns to rising. Hubert Chanson observed the tidal bore of the Dordogne river (France) on 27 September 2000 [1,2,3]. The bore propagates first in the Gironde before separating and continuing both in the Garonne and in the Dordogne. At St Pardon, the tidal bore of the Dordogne river was an undular bore and the photograph shows the arriving bore with kayaks and surfers riding the bore undulations.
This video is associated with a series of photographs published in CHANSON (2004, p. 183) [4], at Earth Science Picture of the Day [5] on 19 Dec. 2001, and in two further websites on tidal bores [6], [7].

References

[1] CHANSON, H. (2001). "Flow Field in a Tidal Bore : a Physical Model." Proc. 29th IAHR Congress, Beijing, China, Theme E, Tsinghua University Press, Beijing, G. LI Ed., pp. 365-373 (ISBN 7-302-04676-X/TV). (CD-ROM, Tsinghua University Press, ISBN 7-900637-10-9.) {http://eprint.uq.edu.au/archive/00000362/}

[2] CHANSON, H. (2005). "Physical Modelling of the Flow Field in an Undular Tidal Bore." Jl of Hyd. Res., IAHR, Vol. 43, No. 3, pp. 234-244 (ISSN 0022-1686). {http://eprint.uq.edu.au/archive/00002474/}

[3] CHANSON, H. (2005). "Tidal Bore of the Dordogne River (France) on 27 September 2000." Flowvis: the Art of Fluid Dynamics, Australian Institute of Physics' (AIP) "Physics for the Nation" Congress, Australian National University (ANU), School of Art, Canberra, Australia, 31 Jan. to 4 Feb. 2005. {http://eprint.uq.edu.au/archive/00002989/}
        Also Flowvis: the Art of Fluid Dynamics, South Australian Museum, Adelaide, 8-25 April 2005.
        Also Flowvis Exhibition, Multimedia art gallery, Frankston, Victoria, 6-20 November 2005. {http://artscentre.frankston.vic.gov.au/Venues/Cube_37/index.aspx}