“Every church is like a stream,” the minister’s speech began, “and each stream flows into a large body of water called the Reformed Church.”
This speech was in support of a motion to radically shift the lines of accountability. Rather than the clear and generally local accountability that has been historic to the Reformed, there has been a movement to make everyone accountable to everyone else, eliminating the clear lines of accountability and risk plunging the whole system into chaos.
“So if one church pollutes their stream,” the minister continued, “it pollutes the whole body, and there is no way that anyone can stop them from polluting in the first place.”
It can sound like a reasonable statement, however, this shows a view of the location of “church” that is not correct.
So when we say “church,” to what are we referring?
Broadly, the term “church” can refer to the “one holy catholic and apostolic church” as the Nicene Creed phrases it. However, we cannot only see the church in this massive and ethereal way, after all, the church is lived out in a real and concrete way, “at the place where it manifests itself in action.” 
In contrast to “monarchical episcopacy” , where the basic manifestation of the church is the diocese and the diocesan bishop’s connection to the Bishop of Rome, the Reformed identify the basic manifestation of the church as a “local church,” that is “a body of baptized Christians meeting regularly in a particular place of worship…properly organized, and is served and governed by a regularly constituted consistory” (BCO, 1.I.1.1).
In the Reformed tradition, then, the church is fundamentally local. That is, the church is gathered around pulpit, table, and font.
Indeed, for the Reformed, the local church — which is served by the offices, and which has the ministry of the Word and sacraments — is truly an ecclesia completa, or complete church. However, this does not mean that local churches are completely independent or autonomous. Just as foundational as the local church being an ecclesia completa is the principle that no church stands on its own, and that there must be real and concrete ties with other churches.  Indeed, churches cannot be autonomous and completely independent not sharing real relationships with one another, for “the need for relationships…stems from the essential nature of the church.” 
Or, put another way, “every local church is therefore simultaneously an independent manifestation of the body of Christ and part of a larger whole.” 
Therefore, in the Reformed Church, when one speaks of the church there is the simultaneous reference to the universal church and the local church. One does not refer–at least primarily–to a communion. In this, the Reformed navigate between Scylla and Charybdis, between episcopacy and independency.
But what about the greater assemblies? Do not the greater assemblies have a greater authority than the lesser assemblies? Not at all. The greater assemblies do not have a greater authority or an authority which is different in essence from that which is given to and exercised by the consistory (or classis).  Indeed, “greater assemblies care for the ministry that extends beyond the purview of the lesser assemblies without infringing on the responsibilities of the lesser” (BCO, Preamble, emphasis mine).
The greater assemblies are, then, not in a clearer sense more “church” than the local church. “Church” begins with the local church and then moves up, not the other way around.
So to return to the faulty analogy quoted at the beginning. This minister got the order and movement backwards. The church is not a basin into which the ministries of the local churches flow. The church is most clearly church around pulpit, table, and font — around Word and sacrament as it gives life to the community, and therefore to the world.
So to offer a more correct analogy, there is a rich and limitless aquifer of Scripture, as well as doctrine, liturgy, and polity which helps us to interpret and live out God’s desires through Scripture. And it is this aquifer which feeds countless streams which spread out and give growth and nourishment and life to the dry and parched land.
The streams flow out, not in. Therefore, yes, the streams are the churches. But the flow is not into a basin, but out into the world.
 Bavinck, Herman. Reformed Dogmatics. Grand Rapids: Baker Academic. 4.373.
 Long, Edward Le Roy. Patterns of Polity: Varieties of Church Governance. Cleveland, OH: Pilgrim Press, 2001.
 Plaatjies van Huffel, Mary-Anne. “The Relevance of Reformed Church Polity Principles: Revisiting the Concept.” In Protestant Church Polity in Changing Contexts I, edited by Allan J. Janssen and Leo J. Koffeman, 29-47. Zurich: LIT Verlag, 2014.
 Coertzen, Pieter. Church and Order: A Reformed Perspective. Leuven: Peeters, 1998.
 Bavinck, 4.374.
Koffeman, Leo J. In Order to Serve: An Ecumenical Introduction to Church Polity. Zurich: LIT Verlag, 2014; Berkhof, Louis. Systematic Theology.