<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def streamDefByThreshold(self, stream_raster_grid, threshold, contributing_area_grid, mask_grid=None, ):
 """ Calculates the stream definition by threshold. """

        log("PROCESS: StreamDefByThreshold")
        self.stream_raster_grid = stream_raster_grid

        # Construct the taudem command line.
        cmd = [os.path.join(self.taudem_exe_path, 'threshold'),
               '-ssa', contributing_area_grid,
               '-src', self.stream_raster_grid,
               '-thresh', str(threshold),
               ]

        if mask_grid:
            cmd += ['-mask', mask_grid]

        self._run_mpi_cmd(cmd)

        # create projection file
        self._add_prj_file(contributing_area_grid,
                           self.stream_raster_grid)