  The Cl1604 supercluster at z=0.9 is one of a small handful of such structures
discovered in the high redshift universe, and is the first target observed as
part of the Observations of Redshift Evolution in Large Scale Environments
(ORELSE) Survey. To date, Cl1604 is the largest structure mapped at z~1, with
the most constituent clusters and the largest number of spectroscopically
confirmed member galaxies. In this paper we present the results of a
spectroscopic campaign to create a three-dimensional map of Cl1604 and to
understand the contamination by fore- and background large scale structures.
Combining new Deep Imaging Multi-object Spectrograph observations with previous
data yields redshifts for 1,383 extragalactic objects in a ~ 0.08 sq. deg
region, 449 of which are supercluster members. We examine the complex three
dimensional structure of Cl1604, providing velocity dispersions for eight of
the member clusters and groups. Our extensive spectroscopic dataset is used to
examine potential biases in cluster velocity dispersion measurements in the
presence of overlapping structures and filaments. We also discuss other
structures found along the line-of-sight, including a filament at z=0.6 and two
serendipitously discovered clusters/groups at z~1.2.
