Femtosecond lasers have been successfully used to perform refractive surgery, by cutting within the bulk of the corneal tissue. As a side effect to the laser cutting there, a streak-like discoloration is observed in histological sections above and below the cutting plane, incident with the direction of laser propagation. These streak-shaped alterations of tissue are believed to originate from low free-electron densities not sufficient to cause optical breakdown. To understand the generation of the streaks, the nonlinear interaction of ultra-short laser pulses with water, as an approximation to corneal tissue, is simulated numerically using a model that simultaneously describes both the nonlinear pulse propagation and the generation of free electrons. The model is used to calculate spatial free electron density distributions generated by ultra-short pulses. Areas of high free-electron density correspond to optical breakdown, whereas areas of low density can be related to streaks. The numerical code can be adapted to practically any transparent dielectric Kerr medium, whose nonlinear optical parameters are known.