diff --git a/compiler/kernel/vop.go b/compiler/kernel/vop.go
index 869cc94a34..cb7cb4ef0b 100644
--- a/compiler/kernel/vop.go
+++ b/compiler/kernel/vop.go
@@ -84,7 +84,7 @@ func (b *Builder) compileVamFork(fork *dag.Fork, parents []vector.Puller) ([]vec
 	for _, seq := range fork.Paths {
 		var parent vector.Puller
 		if f != nil && !isEntry(seq) {
-			parent = f.AddExit()
+			parent = f.AddBranch()
 		}
 		exit, err := b.compileVamSeq(seq, []vector.Puller{parent})
 		if err != nil {
diff --git a/runtime/vam/op/fork.go b/runtime/vam/op/fork.go
index 5a2976776d..343b25e2c5 100644
--- a/runtime/vam/op/fork.go
+++ b/runtime/vam/op/fork.go
@@ -2,97 +2,29 @@ package op
 
 import (
 	"context"
-	"sync"
 
 	"github.com/brimdata/super/vector"
 )
 
 type Fork struct {
-	ctx    context.Context
-	parent vector.Puller
-
-	branches []*forkBranch
-	nblocked int
-	once     sync.Once
+	router *router
 }
 
 func NewFork(ctx context.Context, parent vector.Puller) *Fork {
-	return &Fork{
-		ctx:    ctx,
-		parent: parent,
-	}
-}
-
-func (f *Fork) AddExit() vector.Puller {
-	branch := &forkBranch{f, make(chan result), make(chan struct{}), false}
-	f.branches = append(f.branches, branch)
-	return branch
-}
-
-func (f *Fork) run() {
-	for {
-		if f.nblocked == len(f.branches) {
-			// Send done upstream.
-			if _, err := f.parent.Pull(true); err != nil {
-				for _, b := range f.branches {
-					select {
-					case b.resultCh <- result{nil, err}:
-					case <-f.ctx.Done():
-					}
-				}
-				return
-			}
-			f.unblockBranches()
-		}
-		vec, err := f.parent.Pull(false)
-		for _, b := range f.branches {
-			if b.blocked {
-				continue
-			}
-			select {
-			case b.resultCh <- result{vec, err}:
-			case <-b.doneCh:
-				b.blocked = true
-				f.nblocked++
-			case <-f.ctx.Done():
-				return
-			}
-		}
-		if vec == nil && err == nil {
-			// EOS unblocks all branches.
-			f.unblockBranches()
-		}
-	}
+	f := &Fork{}
+	f.router = newRouter(ctx, f, parent)
+	return f
 }
 
-func (f *Fork) unblockBranches() {
-	for _, b := range f.branches {
-		b.blocked = false
-	}
-	f.nblocked = 0
+func (f *Fork) AddBranch() vector.Puller {
+	return f.router.addRoute()
 }
 
-type forkBranch struct {
-	fork     *Fork
-	resultCh chan result
-	doneCh   chan struct{}
-	blocked  bool
-}
-
-func (f *forkBranch) Pull(done bool) (vector.Any, error) {
-	f.fork.once.Do(func() { go f.fork.run() })
-	if done {
-		select {
-		case f.doneCh <- struct{}{}:
-			return nil, nil
-		case <-f.fork.ctx.Done():
-			return nil, f.fork.ctx.Err()
+func (f *Fork) forward(vec vector.Any) bool {
+	for _, r := range f.router.routes {
+		if !r.send(vec, nil) {
+			return false
 		}
 	}
-	select {
-	case r := <-f.resultCh:
-		return r.vector, r.err
-	case <-f.fork.ctx.Done():
-		return nil, f.fork.ctx.Err()
-	}
+	return true
 }
diff --git a/runtime/vam/op/router.go b/runtime/vam/op/router.go
new file mode 100644
index 0000000000..c7c4e6f200
--- /dev/null
+++ b/runtime/vam/op/router.go
@@ -0,0 +1,113 @@
+package op
+
+import (
+	"context"
+	"sync"
+
+	"github.com/brimdata/super/vector"
+)
+
+type forwarder interface {
+	forward(vector.Any) bool
+}
+
+type router struct {
+	ctx       context.Context
+	forwarder forwarder
+	parent    vector.Puller
+
+	routes   []*route
+	nblocked int
+	once     sync.Once
+}
+
+func newRouter(ctx context.Context, f forwarder, parent vector.Puller) *router {
+	return &router{ctx: ctx, forwarder: f, parent: parent}
+}
+
+func (r *router) addRoute() vector.Puller {
+	route := &route{r, make(chan result), make(chan struct{}), false}
+	r.routes = append(r.routes, route)
+	return route
+}
+
+func (r *router) run() {
+	for {
+		if r.nblocked == len(r.routes) {
+			// Send done upstream.
+			if _, err := r.parent.Pull(true); err != nil {
+				for _, route := range r.routes {
+					select {
+					case route.resultCh <- result{nil, err}:
+					case <-r.ctx.Done():
+					}
+				}
+				return
+			}
+			r.unblockBranches()
+		}
+		vec, err := r.parent.Pull(false)
+		if vec != nil && err == nil {
+			if !r.forwarder.forward(vec) {
+				return
+			}
+			continue
+		}
+		for _, route := range r.routes {
+			if !route.send(vec, err) {
+				return
+			}
+		}
+		if vec == nil && err == nil {
+			// EOS unblocks all branches.
+			r.unblockBranches()
+		}
+	}
+}
+
+func (r *router) unblockBranches() {
+	for _, route := range r.routes {
+		route.blocked = false
+	}
+	r.nblocked = 0
+}
+
+type route struct {
+	router   *router
+	resultCh chan result
+	doneCh   chan struct{}
+	blocked  bool
+}
+
+func (r *route) Pull(done bool) (vector.Any, error) {
+	r.router.once.Do(func() { go r.router.run() })
+	if done {
+		select {
+		case r.doneCh <- struct{}{}:
+			return nil, nil
+		case <-r.router.ctx.Done():
+			return nil, r.router.ctx.Err()
+		}
+	}
+	select {
+	case r := <-r.resultCh:
+		return r.vector, r.err
+	case <-r.router.ctx.Done():
+		return nil, r.router.ctx.Err()
+	}
+}
+
+func (r *route) send(vec vector.Any, err error) bool {
+	if r.blocked {
+		return true
+	}
+	select {
+	case r.resultCh <- result{vec, err}:
+	case <-r.doneCh:
+		r.blocked = true
+		r.router.nblocked++
+	case <-r.router.ctx.Done():
+		return false
+	}
+	return true
+}