You can pan it, another option build a sluice box, you can use it to check for values, or leave it in the ditch to collect anything moving down channel. if you have some old pumps a dredge is really easy to build, with some welding skill suction nozzles are easy also, first try panning, if nothing found then no need to waste more time,
If you do not have a gold pan, I have used old hub caps, or pans like aluminum skillets, cut a few very small pieces of lead, smaller than BB's, put them into the bottom of your pan, fill pan full or gravel, sand mud and water, make circle motion with pan swishing around materials, this will help lighter materials to rise in water and heavy materials go to bottom of pan, now tilt pan forward giving it a couple of shakes sideways back and forth, this gets gold down into lowest portion of pan, now dump the water and about half of the materials out of the pan, refill with water and repeat, removing larger rocks by hand, now when just black sand few spoons of materials left, you can rock water back and forth over the top of the black sand (get water on top of sand without moving sand, then move water away faster carrying some of sand in movement of water), this will reveal your lead and any gold or silver from your ditch.
Irrigation ditches are not likely to carry much gold unless they picked it up from ditch as it traveled through to you or from erosion of mountain rains into the ditch, they normally move slow and not likely to flood, water is usually brought out of river, the can have some fine flakes here in my part of the country, but streams rivers and ancient dry river beds are better to work for values, here in the mountains I have seen irrigation ditches dug for twenty miles and further by miners to bring water to their workings. these ditches dug through ground that contained gold, they did not pan these ditches the water had more value delivered to the mine site, these ditches do show some color (gold) when panned.
gold will move to bottom (down through gravel rock sand and mud) of river until it reaches bedrock, here it will roll down river until caught in cracks in bedrock, in the river bedrock can be one foot down or twenty or more feet down, so panning surface gravel and sand will usually only turn up a few small flakes of gold just about every panfull around here, you have to get down to bedrock to do any good.
