High-intensity lasers that strike the surface of a material that is transparent will release power into the material. If the absorption of the incident light can be described by the Beer-Lambert law, it is possible to model this power deposition using the core functionality of COMSOL Multiphysics.  You can also get more information about river hydraulics online.

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The Beer-Lambert Law and Material Heating

When light is incident upon a semi-transparent material, some of the energy will be absorbed by the material itself. If we can assume that the light is the single wavelength, collimated (such as from a laser), and experiences very minimal refraction, reflection, or scattering within the material itself, then it is appropriate to model the light intensity via the Beer-Lambert law. 

Implementation in COMSOL Multiphysics

It depicts a solid cylinder of material (20 mm in diameter and 25 mm in length) with a laser incident on the top. To reduce the model size, they exploit symmetry and consider only one-quarter of the entire cylinder. 

The partition of the domain is up into two volumes. These volumes will represent the same material, but we will only solve the Beer-Lambert law on the inside domain — the only region that the beam is heating up.