There are quite a few ways to access this location but perhaps the easiest (and its approximately a 1 mile walk to and from the reservoir) is off the intersection of Paraiso and Arroyo Vista in Rancho Santa Margarita.  The details below explain how to get to the reservoir based on this access point.  Alternatively, you may wish to explore the entrance on Google Maps from Olympiad in Mission Viejo at the back end of Beebe Park.

The GPS coordinates of the reservoir on the Arroyo Trabuco Trail are approximately at these GPS coordinates:  33.609333, -117.627315

The trail head and parking at Paraiso and Arroyo Vista intersection are at these coordinates:  33.609333, -117.627315

Simply copy and paste the reservoir or trail head coordinates at Google Maps

I’ve photos below to offer a rough sense of the terrain.  Please bear in mind, you may come across Bobcats, Southern Pacific Rattlesnake (both of which I noted on 7-14-14).  There’s also a probable stream crossing depending on what time of year you seek out this trail.  Photos below were taken mid July 2014.  The stream was low so crossing was no issue and was approximately 1-2 inches in height (photos below also)

an adult seen just before the landmark shown below
an adult seen just before the landmark shown below
Arroyo Trabuco Trail entrance
Entrance off Paraiso and Arroyo Vista.  Park on street just beyond this gate
Just past main entrance.  You are bearing left here.  Walk down slope.
Just past main entrance. You are bearing left here. Walk down slope.

Bear right at the trail sign, then a quick left and walk down this slope.  Head south (left).

Arroyo Trabuco Trail-heading southArroyo Trabuco Trail heading south
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Arroyo Trabuco TrailApproaching land mark on left.
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Arroyo Trabuco Trail - RSMBear right at land mark, then stay left

Descend down the trailDescend down this trail

Continue descending
Continue descending
Arroyo Trabuco Trail
Heading toward stream crossing
Stream Crossing
Stream Crossing
facing backward, the stream was just beyond the foliage shroud
facing backward, the stream was just beyond the foliage shroud
Facing back toward stream.  At right is the berry tree
Facing back toward stream. At right is the berry tree

 

photo of the reservoir
photo of the reservoir