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In order to comprehend potential changes in the hydrodynamics of flow under downward seepage condition when water is extracted from a threshold alluvial channel, investigations in a laboratory flume were carried out. Experiments in the present study, were organized in two categories: in absence of downward seepage and in presence of downward seepage. Increased in transport of bed-material in the channel was observed on introduction of downward seepage. Instantaneous velocity samples were collected employing an acoustic Doppler velocimeter (ADV) to calculate various length scales of turbulent motions such as: integral length scale, Taylor microscale, and Kolmogorov's microscale. An increase in the values of these length scales has been observed under the influence of downward seepage. Three sub-ranges in the velocity power spectra both for seepage and no-seepage situations are confirmed by the results of the spectral analysis in the channel near-bed area. Toward the channel bed, spectral densities were found increasing for both types of experiments. At every measurement location, velocity power spectra of the seepage experiment shifted toward higher spectral densities and higher frequencies than the power spectra of the no-seepage experiment. Moreover, the power spectra of seepage experiment were found flatter than that of the no-seepage experiment. This flatness in the velocity power spectra indicates an increased bed roughness when a threshold alluvial channel is subjected to downward seepage.