I will present a new instability mechanism that affects shear flows when these are
stably stratified in density along the vertical direction orthogonal to the horizontal shear plane.

Stratified shear flows are ubiquitous in nature and we may think to water flows in submarine canyons,
to winds in deep valleys, to currents along sea shores or to laminar flows in canals where density stratification can be due to temperature or salinity gradients. Our study follows recent investigations on instabilities in stratified rotating or non rotating shear flows: the stratorotational instability or the stratified boundary layer instability where it was shown that these instabilities belong to a class of instabilities caused by the resonant interaction of Doppler shifted internal waves. Our laboratory experiments for Plane Couette and Plane Poiseuille flows show in both cases the appearance of braided wave patterns when the Reynolds and Froude numbers are above a threshold. The non linear saturation of the instability leads to a meandering in the horizontal plane arranged in layers stacked along the vertical direction.