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In this paper, we analyze the high-resolution UV spectra for a C1.7 solarflare (SOL2017-09-09T06:51) observed by the \textit{Interface Region ImagingSpectrograph} (\textit{IRIS}). {We focus on the spectroscopic observations atthe locations where the cool lines of \ion{Si}{4} 1402.8 \AA\ ($\sim$10$^{4.8}$K) and \ion{C}{2} 1334.5/1335.7 \AA\ ($\sim$10$^{4.4}$ K) reveal significantredshifts with Doppler velocities up to $\sim$150 km s$^{-1}$.} These redshiftsappear in the rise phase of the flare, then increase rapidly, reach the maximumin a few minutes, and proceed into the decay phase. Combining the images from\textit{IRIS} and Atmospheric Imaging Assembly (AIA) on board the {\em SolarDynamics Observatory} ({\em SDO}), we propose that the redshifts in the coollines are caused by the downflows in the transition region and upperchromospheric layers, which likely result from a magnetic reconnection leadingto the flare. In addition, the cool \ion{Si}{4} and \ion{C}{2} lines showgentle redshifts (a few tens of km s$^{-1}$) at some other locations, whichmanifest some distinct features from the above locations. This is supposed tooriginate from a different physical process.

Spectroscopic Observations of High-speed Downflows in a C1.7 Solar Flare