A method for measuring a mean surface velocity of a fluid flowing in a channel, comprising: transmitting with at least one beam (102; 202; 402; 602; 702), using a transmitter of a device (101; 201, 401; 601; 701), directed energy carrying a signal toward a surface of a fluid (104; 204, 404; 604; 704) in a fluid channel (103; 203; 403; 603; 703), so as to produce one or more reflections from the fluid surface that substantially span the width of the fluid channel (103; 203; 403; 603; 703); detecting, by at least one receiver of the device (101; 201, 401; 601; 701), one or more received signals associated with the one or more reflections so produced; and determining, based upon the transmitted and received signals, a surface velocity to be measured using a processor of the device (101; 201, 401; 601; 701), wherein the determining comprises identifying, for each of the one or more received signals, a surface velocity corresponding to the location of the corresponding received signal and determining a mean surface velocity from the surface velocities each of the one or more received signals; characterized in that, at least one beam characteristic is adjusted based on a distance from the device (101; 201, 401; 601; 701) to the fluid surface, wherein one of the at least one beam characteristic that is adjusted comprises a width of the beam (102; 202; 402; 602; 702) and wherein adjustment of the width of the at least one beam comprises maintaining a width of the beam contacting the surface of the fluid (104; 204, 404; 604; 704) by widening the beam as the distance from the device to the fluid surface is lessened, whereby the width of the beam narrows when the distance from the device increases. transmitting with at least one beam (102; 202; 402; 602; 702), using a transmitter of a device (101; 201, 401; 601; 701), directed energy carrying a signal toward a surface of a fluid (104; 204, 404; 604; 704) in a fluid channel (103; 203; 403; 603; 703), so as to produce one or more reflections from the fluid surface that substantially span the width of the fluid channel (103; 203; 403; 603; 703); detecting, by at least one receiver of the device (101; 201, 401; 601; 701), one or more received signals associated with the one or more reflections so produced; and determining, based upon the transmitted and received signals, a surface velocity to be measured using a processor of the device (101; 201, 401; 601; 701), wherein the determining comprises identifying, for each of the one or more received signals, a surface velocity corresponding to the location of the corresponding received signal and determining a mean surface velocity from the surface velocities each of the one or more received signals; characterized in that, at least one beam characteristic is adjusted based on a distance from the device (101; 201, 401; 601; 701) to the fluid surface, wherein one of the at least one beam characteristic that is adjusted comprises a width of the beam (102; 202; 402; 602; 702) and wherein adjustment of the width of the at least one beam comprises maintaining a width of the beam contacting the surface of the fluid (104; 204, 404; 604; 704) by widening the beam as the distance from the device to the fluid surface is lessened, whereby the width of the beam narrows when the distance from the device increases.