I think it has to do with silt building up in the riverbed.

Note the article at: http://www.springerlink.com/content/g864230541428514/
The abstract in Spanish uses "laminación" for 'buildup'.

Abstract: Ripple-laminated sandy deposits at shallow water depths in four rivers on Jameson Land, .... build-up were better bleached than those deposited during .... the equivalent doses of the modern river bed sediments ...
www.geochronometria.pl/pdf/geo_26/Geo26_01.pdf

Feb 6, 2008 ... Over time, the levees build up and provide a higher bank for the ... 2) coarsest sediment may be lower flat laminated if flow speeds are not ...
https://www.geology.ucdavis.edu/.../Rivers-MeanderingandBrai...

In each development of concentrically laminated sediment investigated on Andros ..... the river bed. They are found to contain Oscillatoria, Nostoc, ..... (6) Any particular species of alga may enter into, and help to build up, ...
www.jstor.org/stable/92164

During the falling stage, riverbed macroforms and mesoforms formed under ..... Cheel, R.J. 1990, Horizontal lamination and the sequence of bed phases and ... Log Jams and Flood Sediment Buildup Caused Channel Abandonment and Avulsion in ...
geology.geoscienceworld.org/cgi/content/full/37/7/623