I don't know that lake but that sonar is more than capable.
Consider adjusting some settings and increase sensitivity.
Also I would try scanning around 100 feet in 455 khz. Mega is great but it does not reach as far as the lower frequency. Once you see something then crank it back up to mega and do a closer scan like 30 feet out.
Don't be afraid to sink your own piles near creek channels and points. You can save yourself some time in the long run.
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