Morning comes up the Annisquam on the tide, and the marsh grass along Riverdale goes silver before it goes green. Benco Landscaping Inc. works these river-side yards in Gloucester — mowing and edging, trimming the borders, mulching beds, and hauling off the debris a tidal wind piles against a fence. Under the A. Piatt Andrew bridge the current runs both directions in a single day, and the lawns above it hold their line regardless. There is real satisfaction in the last pass of a mower, when the stripes finally square up with the road. Cape Ann soil lies thin over ledge, so every good lawn here is earned an inch at a time.