It looks like you are telling Arc that you are trying to input a z-factor in the slope tool, but not actually defining the variable. Removing the extra comma from that line of code should help, unless of course you do want to specify the variable in which case you need to put something in there after the comma:
out_slope = Slope(input_raster, out_measurement )
or
out_slope = Slope(input_raster, out_measurement, slope )
As @dchaboya has pointed out, you'll need to explicitly cast the parameter-as-text variables as integers or your script may not work as expected:
mean_discharge = int(arcpy.GetParameterAsText(2))
input_timestep = int(arcpy.GetParameterAsText(3))