  We study the dynamics of low-lying cool loop systems for three datasets as
observed by the Interface Region Imaging Spectrograph (IRIS). Radiances,
Doppler shifts and line widths are investigated in and around observed cool
loop systems using various spectral lines formed between the photosphere and
transition region (TR). Footpoints of the loop threads are either dominated by
blueshifts or redshifts. The co-spatial variation of velocity above the
blue-shifted footpoints of various loop threads shows a transition from very
small upflow velocities ranging from (-1 to +1) km/s in the Mg\,{\sc ii} k line
(2796.20~\AA; formation temperature: log (T/K) = 4.0) to the high upflow
velocities from (-10 to -20) km/s in Si\,{\sc iv}. Thus, the transition of the
plasma flows from red-shift (downflows) to the blue-shift (upflows) is observed
above the footpoints of these loop systems in the spectral line C\,{\sc ii}
(1334.53~\AA; \log (T/K) = 4.3) lying between Mg\,{\sc ii} k and Si\,{\sc iv}
(1402.77~\AA; log (T / K) = 4.8). This flow inversion is consistently observed
in all three sets of the observational data. The other footpoint of loop system
always remains red-shifted indicating downflowing plasma. The multi-spectral
line analysis in the present paper provides a detailed scenario of the plasma
flows inversions in cool loop systems leading to the mass transport and their
formation. The impulsive energy release due to small-scale reconnection above
loop footpoint seems to be the most likely cause for sudden initiation of the
plasma flows evident at TR temperatures.
